Fix keyboard shortcut conflicts

[bnquon]

when setting a shortcut that is already being used, it now asks if you want to move it to the new action.

if you choose Move shortcut, the shortcut is cleared from the old action and assigned to the new one. choosing Cancel leaves both actions unchanged.

inspired by @EXOTARA's approach in the original issue's thread.

(Since filming demo i updated the capitalization of the dialog title and action)

zen-issue-14492-demo.mp4

testing

• tested moving a conflicting shortcut
• tested cancelling the prompt and confirmed both shortcuts stayed unchanged
• checked that the shortcut changes were saved after restarting Zen and opening a new Settings tab
• tested on macOS

note

there seems to be an existing issue where restored Settings tabs show stale shortcut values after restarting Zen. closing the restored Settings tab and opening a new one shows the correct saved values. I was also able to reproduce this with normal non-conflicting shortcut changes on the dev branch, so it doesn’t appear to be related to this change.

Closes #14492

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