Fix #20826 - Numpad numbers no longer work to select note duration.

Fixed by blocking the creation (and pooling) of QAction creation for shortcuts with multiple alternatives in different states (like the 'common' and TAB-specific variants of "pad-note-..." shortcuts)

A rather detailed description of the implementation (regarding both the original, faulty, implementation and this fix) has been added to the mscore/shortcut.h file
Fix crash when displaying shortcut preferences
Add support for "NumPad" qualifier in textual representations of key sequences both in dlg boxes and in xml files.

Also allows to remove the <code> tag from shortcuts.xml
Merge pull request #315 from mgavioli/Fix_20826_Numpad_note_duration_shortcuts

Fix #20826 - Numpad numbers no longer work to select note duration.