Draft: macos: Drag and drop support

Arjan Molenaarrequested to merge
amolenaar/gtk:macos-dragimage into main

This MR adds DnD to macOS. It also contains some work @chergert already did earlier last year.

Although DnD works, and does not seem to crash, there's one caveat: while a DnD action is preformed, the application window is frozen. I haven't figured out how to fix that.

Fixes #3423 (closed).

Edited by Arjan Molenaar

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