feat: add move to trash to the file menu

[itsjoshpark]

• I have read CONTRIBUTING.md

Description:

Finishing a video and wanting it gone meant switching to the Finder to find it. File > Move to Trash now trashes the playing file, closes the player window and returns to the welcome window. There is no keyboard shortcut.

Holding Option turns the item into Delete Immediately.... It asks first, the way the Finder does (“Are you sure you want to delete “file.mkv”?” / “This item will be deleted immediately. You can’t undo this action.”), and then deletes the file without going through the Trash.

• The file's recents entry is removed too, so the welcome window doesn't list a file that is gone.
• The welcome window opens before the player closes, since closing the last window quits the app.
• A file that can't be removed, such as one on a volume with no Trash, leaves the player open and shows an alert with the reason.
• The confirmation and the failure are SwiftUI alerts held by PresentationModel, so they share the one alert slot with the conversion and recent-file alerts. Both items are disabled while any sheet or alert is up, as the Playback items are.

Testing

• Unit tests pass, including a new PresentationModelTests case for the removal alert's slot.
• UI tests pass for Move to Trash, and for Delete Immediately with both Cancel and Delete.
• Not checked by hand yet: that the menu title visibly flips when Option is held, and which button Return presses in the confirmation.