MUS-107 Added NoteFlag

- AbstractDrawerTest: added assert on teardown
- Added enum NoteFlag (no flag, single flag, double flag)
- Refactored NoteLength (added getFlag)
- Added missing NoteSymbol tests
- Deleted a PianoActivityUiTest. It is broken because of the variable
note length change, but this feature will be gone in an upcoming Jira
issue anyway
- Added NoteFlagDrawer