Include all shortcuts in the Shortcuts preferences panel, even if they can't be changed

### Your idea There are a few hardcoded (I assume) shortcuts that are very difficult to discover, because they aren't in the Shortcuts window. Ones that I'm aware of: * Tab: move between grab handles an element * Ctrl-shift-Del / Cmd-shift-backspace: insert a full-bar rest (I'm sure there are others) Even if they can't be modified for some reason, it would be very helpful for discoverability if these were listed on the Shortcuts page. (The "Define..." and "Clear" and "Reset to default" buttons can be greyed out in that case). ### Problem to be solved Discoverability. I assumed there was no keyboard way to navigate to grab handles until I asked in the Discord. I only learnt that it was possible to create full bar rests by randomly coming across #28682. Whereas I had looked for both of those within the Shortcuts UI. ### Prior art _No response_ ### Additional context _No response_ ### Checklist - [x] This request follows the <a href="https://github.com/musescore/MuseScore/wiki/Reporting-bugs-and-issues" target="_blank">guidelines</a> for reporting issues - [x] I have verified that this feature request has not been logged before, by searching the <a href="https://github.com/musescore/MuseScore/issues" target="_blank">issue tracker</a> for similar requests