Pie menu for Rhythm Maker

[pikurasa]

Based on a discussion we had in our meeting last Sunday, I think we should implement this:

1. In easy mode, when a user clicks the number section in Rhythm Maker, they should get a pie menu with options 2, 3, and 4.
2. In advanced mode, when a user clicks the number section in Rhythm Maker, they should get a pie menu with options 2, 3, 4, 5, and 7.

Screenshot of Rhythm Maker widget (the pie menu would initiate by clicking where the "2" is):

A user may still type in other values, similar to how you can type other values for note value (although I'm not sure if we want the option to type for mobile).