macos: Provide minimal menu for native file dialog

Arjan Molenaarrequested to merge
amolenaar/shortcuts-in-native-windows into main

This allows us to support common key combos, such as for cut/copy/paste.

Fixes #6829 (closed).

The solution replaces the main menu when a native open/save dialog is shown.

Of course this works best if the file dialog is a modal.

Edited by Arjan Molenaar

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