GTK: keyboard action to move the focused split into a specific tab

[ignatremizov]

Hi! Mitchell mentioned in #1525 that keyboard bindings for split/tab moves should go to a new discussion, so this is that follow-up for GTK.

I implemented a GTK-side action path inspired by Windows Terminal for moving the focused split into another existing tab and merging it into the destination tab's split tree:

• bindable action: move_split_to_tab:N
• command palette entries for tabs 1 through 9
• default Linux shortcuts: Ctrl+Alt+Shift+1..9
• behavior: remove the focused surface from the source tab, merge it into the destination tab as a split, close the source tab if it becomes empty, and focus the moved surface

This is intentionally keyboard/command driven, not drag-and-drop UI.

This seems adjacent to the earlier macOS mouse-side work:

• macOS: Add ability to change split order #1525
• macOS: Split drag to create new tabs #10092
• macOS: allow to pop surface into a new tab #9457
• GTK mouse follow-up: GTK: Implement split drag and drop to reorder #10224

The user-facing workflow is basically the missing keyboard counterpart to split drag/tab movement. Example use cases:

• move the active pane into tab 3
• reorganize panes across tabs without using the mouse
• keep a stable numbered tab layout and send the current split to a known destination

Questions worth discussing:

1. Is move_split_to_tab:N the right action shape, or should this be a more generic move/detach action with a picker?
2. Should numbered destination-tab actions exist by default, or only as bindable actions without defaults?
3. For parity, should there also be a reverse operation for turning the active tab/surface into a split in another tab?
4. Should GTK eventually also support the mouse/drag variant, with this action as the non-UI backend?

If useful, I would be happy to open a PR with the GTK implementation and default bindings.

[gozes]

Given you can change split focus with Super+ctrl+[ maybe swap split should be Super+ctrl+j?