Allow grace notes to be attached to rests

In MuseScore, grace notes that occur immediately prior to a barline are entered using the 'grace note after' element, but that element can only be attached to notes, not rests, which means that it is not possible to enter a sequence such as this where the final duration element before the barline is a rest:

Extract from Liza Lehmann's - 2 Seal Songs, No. 2 - "You mustn't swim till you're six weeks old".

Similarly, it would not be possible to enter a 'grace note before' that occurs at the beginning of a bar if the first duration element is a rest, though this case is probably much more rare.

Workaround: The workaround in both cases is to attach the grace note to an invisible non-playing note in another voice.