Desktop: Rich Text Editor: Fix cut, copy, paste, and select all menu items

[personalizedrefrigerator]

Summary

This pull request fixes an issue in which the 'cut', 'copy', 'paste', and 'select all' menu items were always disabled in the Rich Text Editor.

This issue was originally reported on the Joplin Forum.

Note

• This pull request targets release-3.5, but perhaps instead it should target dev. At least on Linux, the cut/copy/paste menu items seem to also be broken in the Rich Text Editor in v3.4.
• The upstream issue mentions that the cut/copy/paste keyboard shortcuts also no longer work. This isn't something I have been able to replicate locally (testing on Fedora 43/Linux), but might be a regression specific to MacOS. By enabling the cut/copy/paste menu items, this pull request may also fix the copy/paste keyboard shortcuts on MacOS.

Testing plan

Fedora 43:

1. Open a Markdown note in the Rich Text Editor.
2. Verify that the cut, copy, paste, and select all items in the "Edit" menu are enabled and can be used.
3. Repeat steps 1-2 for an HTML note.

Screen recording:

Screencast.from.2026-01-15.12-52-03.webm

[personalizedrefrigerator]

Not resolved in this pull request.