Fix #20915 - Weird behaviour of "Reset to default" buttons in "Preference | Shortcuts"

The loadDefaultShortcuts() function in mscore/shortcut.cpp was not populating the list it returns.

Also:
- Made labels for "Reset to Default" buttons more explicit.
- Added (temporarily?) support for the &lt;code&gt; tag to loadDefaultShortcuts(), until the pull request with "NumPad" support is merged (https://github.com/musescore/MuseScore/pull/315), which will make the &lt;code&gt; tag obsolete.