uix:ActionBar make items added to ActionOverflow persistant in the
overflow dropdown. Minor bug fixes.
uix:ActionBar fix group_index to be calculated after all groups have
been added
uix:ActionBar allow changing of proeprties like color, mipmap, border
through the ActionItem.
Merge pull request #1399 from kivy/action_overflow

uix:ActionBar make items added to ActionOverflow persistant in the overflow dropdown. Minor bug fixes.