Handle disabled state for dropdowns
Handle disabled state for dropdowns

The disabled state is supported in two ways:

- A disabled dropdown's value cannot be changed using keyboard actions
  (up/down arrow and Home/End).
- Additionally, following some common UI paradigms out there, clicking
  on a disabled dropdown's header does not expand the dropdown. Visual
  indication of this in the basic control will come in a subsequent
  commit.

ppy/osu-framework,ppy/osu-framework,peppy/osu-framework,ZLima12/osu-framework,ppy/osu-framework,smoogipooo/osu-framework,smoogipooo/osu-framework,peppy/osu-framework,ZLima12/osu-framework,peppy/osu-framework