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 <code> tag to loadDefaultShortcuts(), until the patch with "NumPad" support is merged (pull request 315), which makes the <code> tag obsolete.
Some code clean ups.
Merge pull request #320 from mgavioli/Fix_20915_shortcut_reset_to_default

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