Fix #22870 - No shortcut available to enter rests in TAB's

Fixed by adding a TAB-specific shortcut similar to "rest" (def. '0').

Its default shortcut has been tentatively assigned to ' (apostrophe)
Merge pull request #493 from mgavioli/Fix-22870_rests_cannot_be_entered_in_TABs

Fix #22870 - No shortcut available to enter rests in TAB's