rc clipboard: Support system clipboard interactions

This commit implements an API to interact with the system clipboard.

Similarly to how terminals are picked by `x11.kak`, the
`clipboardcmd_copy` and `clipboardcmd_paste` options detect which tool
should be used to interact with the system clipboard at startup. Only
`xsel` has been tested, but `pbcopy`/`pbpaste` and `wl-copy`/`wl-paste`
will be used if they're installed. These options can also be overridden
by the user to use `xclip` or any other tool they want.

Commands allow copying, pasting before/after, replacing
selections. Data to copy to the clipboard can also be passed directly
to `clipboard-copy`.

A convenience alias forwards `clipboard-paste` to
`clipboard-paste-after` to save users, who expect pasting to always
append, some brain cycles.

The `clipboard-mode` user mode is also declared to accommodate user
mappings, and assigned predictable key mappings.

Savvier users can enable register integration with
`clipboard-enable-register`, which currently only works one way: all
contents assigned to the given register is automatically forwarded to
the system clipboard. Vim uses the `*` and `+` registers, but
punctuation signs are generally associated with “core” primitives
in Kakoune, so using a letter register for this opt-in layer of
abstraction seemed more fitting.

Implementing the register integration in the opposite direction
in a portable way is not self-evident, and thus not implemented by
this patch.

Fixes mawww#3935.

rc/tools/clipboard.kak

@@ -0,0 +1,112 @@
+declare-option -docstring %{
+    The shell command to which data will be piped to copy to the system clipboard
+} str clipboardcmd_copy %sh{
+    for cmd in "xsel -i" "pbcopy" "wl-copy -p"; do
+        readonly program="${cmd%% *}"
+        if command -v "${program}" >/dev/null 2>&1; then
+            printf %s "${cmd}"
+            exit
+        fi
+    done
+}
+
+declare-option -docstring %{
+    The shell command which will paste the system clipboard into the current buffer
+} str clipboardcmd_paste %sh{
+    for cmd in "xsel -o 2>/dev/null" "pbpaste" "wl-paste -p"; do
+        readonly program="${cmd%% *}"
+        if command -v "${program}" >/dev/null 2>&1; then
+            printf %s "${cmd}"
+            exit
+        fi
+    done
+}
+
+define-command -params .. -docstring %{
+    clipboard-copy [<data>…]: copy the given data to the system clipboard
+
+    If no data is passed as argument to the command, the current selection is used
+    If multiple selections/data arguments are to be stored into the clipboard, they are joined with a newline character
+} clipboard-copy %{ evaluate-commands %sh{
+    join() {
+        # NOTE: this is a POSIX way of storing a newline
+        # character into $IFS without spawning a subshell
+        IFS='
+'
+        printf %s "$*"
+    }
+
+    if [ $# -eq 0 ]; then
+        eval "set -- $kak_quoted_selections"
+    fi
+
+    if ! join "$@" | eval "${kak_opt_clipboardcmd_copy}"; then
+        echo fail "Unable to copy data to the system clipboard"
+    fi
+} }
+
+define-command -docstring %{
+    Paste the contents of the system clipboard before the selection
+} clipboard-paste-before %{
+    execute-keys ! <space> %opt{clipboardcmd_paste} <ret>
+}
+
+define-command -docstring %{
+    Paste the contents of the system clipboard after the selection
+} clipboard-paste-after %{
+    execute-keys <a-!> <space> %opt{clipboardcmd_paste} <ret>
+}
+
+define-command -docstring %{
+    Replace the selection with the contents of the system clipboard
+} clipboard-replace %{
+    evaluate-commands -save-regs c %{
+        set-register c %sh{ eval "${kak_opt_clipboardcmd_paste}" }
+        execute-keys \"c R
+    }
+}
+
+define-command -params ..1 -docstring %{
+    clipboard-enable-register [<register>]: automatically copy the contents of the given register to the clipboard, when it is modified
+
+    If no register is passed as argument, the `c` register will be used
+    The built-in copy register can be referred to as `dquote`, instead of `"`
+} -shell-script-candidates %{
+    echo dquote
+} clipboard-enable-register %{ evaluate-commands %sh{
+    reg="${1:-c}"
+
+    # Make sure the register is valid
+    printf 'nop %%reg{%s}\n' "${reg}"
+
+    case "${reg}" in
+        dquote) reg='"';;
+    esac
+
+    # NOTE: The following will break with the closing-brace register
+    printf '
+        try clipboard-disable-register
+        hook -group clipboard-bind-register global RegisterModified %%{%s} %%{
+            clipboard-copy %%reg{%s}
+        }
+    ' "${reg}" "${reg}"
+} }
+
+define-command -docstring %{
+    Disable integration of the system clipboard with the `c` register
+} clipboard-disable-register %{
+    remove-hooks global clipboard-bind-register
+}
+
+# NOTE: Users who discover the command interactively might not want to
+# have to think about the before/after side effects, so assume they expect
+# pasting to always be performed after the selection
+alias global clipboard-paste clipboard-paste-after
+
+declare-user-mode clipboard
+
+map global clipboard y -docstring "Copy selection to clipboard"      ": clipboard-copy<ret>"
+map global clipboard p -docstring "Paste clipboard after selection"  ": clipboard-paste-after<ret>"
+map global clipboard P -docstring "Paste clipboard before selection" ": clipboard-paste-before<ret>"
+map global clipboard r -docstring "Replace selection with clipboard" ": clipboard-replace<ret>"
+map global clipboard x -docstring "Cut selection to clipboard"       ": clipboard-copy; exec d<ret>"