* Gray out status-bar language switcher's "Set as Default" item if the doc's
current language is already the default (i.e. there's no path-specific override).

DropdownButton changes:
* Support disabled items
* Fix bug #8433 (Page Up/Down work oddly) - Only wrap around to other end
of list when using arrow keys. Page Up/Down 'stick' at start/end of list
instead (so they will act like Home/End when there's no scrollbar).
* Add Home/End key support