GTK: move tab to a new window

[jcollie]

Currently (with libadwaita enabled) you can move a tab to another existing window by dragging the tab header from one window to another. I propose adding a keybind action and corresponding context menu entries to move a tab to a new (previously non-existent) window where it would become the first and only tab in that window.

[mitchellh]

I'm supportive of this, and also supportive of making this a generic keybind not specific to GTK (as usual, I can do the macOS work when it comes down to it). It isn't difficult to do on macOS.

What should we name the keybind?

[jcollie]

I was thinking move_tab_to_new_window. Verbose but pretty obvious.

[mitchellh]

#2630