Regroup Rhythms doesn't work for tuplets with nested tuplets

Reproducing the issue:
1. Attach a simple score containing with a tuplet of rests with a nested tuplet of rests.

2. Regroup the score.
3. The score remains unaffected.

However, in the example it should regroup the tuplet to a full 1/4 rests and regroup it with the first half rest.

The root cause is Score::regroupNotesAndRests() only works for tuplets, but not nested tuplets. A good fix would be to create a Tuplet::containsOnlyRests() function which could recursively check where the tuplet contains nested tuplets, and whether all are rests