added support for dragging widgets and app icons to mini screens

- gave mini screens a rounded rect background
- mini screens change color when dragged over
- added a way to specify the active region of dragged items, to allow for clipping of whitespace at borders
- fixed: feedback while dragging was often one cell off
- fixed: bug when adding to almost-full screen
- removed unused classes

Change-Id: I2fbd2aaaee6831b288cca8dec75e7b446068e8d1
12 files changed