Make -currentValueForControlStateKey: public

Useful for subclasses
Merge branch 'btrpopupbutton-alignment' into btrcontrol-state-additions
Merge branch 'master' into btrcontrol-state-additions
Add constants for state keys
Update commment
Merge pull request #48 from ButterKit/btrcontrol-state-additions

Make -currentValueForControlStateKey: public