deleted unused classes

Removed the About Screen and its QrCode. We will add an about screen
later, but right now it is dead code.
Completely rewrote slate handling

The slate is now contained in its own SlateFragment. This way we can
use fragment.setRetainInstance(true) to keep it on screen rotations
(the activity's equivalent of this feature is deprecated) :
-Rewrote Draw Activity, removed the slate and all dead code from it, it
will be easier to maintain that way.
-Added SlateFragment Class.
-removed android:configChanges="orientation|screenSize" from the
manifest since the fragment handles the rotations now.
-added utils.Rotation class to handle SlateRotations.
-Improved brushSize seeker, it is now independant from the screen
density.
resolved merge conflict