implement "Set Touch Bar Key Labels" for iTerm2

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https://iterm2.com/documentation-escape-codes.html

Set Touch Bar Key Labels

You can configure touch bar key labels for function keys and for the "status" button. The code used is:

OSC 1337 ; SetKeyLabel=[key]=[value] ST

Where [key] is one of F1, F2, ..., F24, to adjust a function key label; or it can be status to adjust the touch bar status button. You can also save and restore sets of key labels using a stack. To push the current key labels on the stack use:

OSC 1337 ; PushKeyLabels ST

To pop them:

OSC 1337 ; PopKeyLabels ST

You can optionally label the entry in the stack when you push so that pop will pop multiple sets of key labels if needed. This is useful if a program crashes or an ssh session exits unexpectedly. The corresponding codes with labels are:

OSC 1337 ; PushKeyLabels=[label] ST
OSC 1337 ; PopKeyLabels=[label] ST

Where [label] is an ASCII string that works best if it is unique in the stack.

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See also: touch bar interaction in wx backend
https://github.com/elfmz/far2l/blob/master/WinPort/src/Backend/WX/Mac/touchbar.h

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bool TTYBackend::OnConsoleSetFKeyTitles(const char **titles)
{
    if (!_far2l_tty) {
        const char* term_program = getenv("TERM_PROGRAM");
        if (term_program != NULL && strcmp(term_program, "iTerm") == 0) {
            // ask TTYOutput to send iTerm2 escape sequences
            return true;
        }
        return false;
    } else {
        // ... current code ...
    }

Not sure, iTerm or iTerm2, yet