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

Fix #20826

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.

See: http://musescore.org/en/node/20826

Addendum: added support for "NumPad+" qualifier in textual representations of key sequences (both in dlgs and in shortcuts.xml), removing the need for the &lt;code&gt; tag in shortcuts.xml.