uix:DropDown escape dismisses the dropdown
ClearColor set to transparent

There's already a window ClearColor to provide some constant background color.  If the screen manager contents are transparent, it makes sense for the screen manager background to be transparent.
ClearColor back to (0, 0, 0, 1) until we know more

See discussion in ae64000280eedd0f75d39578631c6c298ca10c62
Merge pull request #1512 from kivy/drop_down_escape

uix:DropDown escape dismisses the dropdown
Merge pull request #5 from kivy/master

Update