Show 'obvious' shortcuts like ctrl+X/ctrl+C/ctrl+V

## Problem

New users aren't idiots, but when they see the super key using the same icon as cmd they will instinctively make the connection, which includes the action bound to cmd.

## Proposal

Add the overarching system shortcuts to the overlay, maybe right at the top since there is a finite amount of them.
(admittedly I would be down for trying to swap the ctrl behaviours to cmd if it's possible in wayland, even if it's handled by GTK it probably wouldn't be impossible to see if they can create a gsetting to swap it)

Review the shortcut overlay implementation and determine where system-wide shortcuts such as Ctrl+X, Ctrl+C, and Ctrl+V are defined or displayed. The issue does not name files or tests; done would require an agreed list of shortcuts and confirmation that the overlay presents them correctly.

Written by the indexing model from the issue text.