Point refactoring, Drawable Element refactoring

 - All positioning is now done via Point objects.
 - There is now a central interface above all drawables, called
   DrawableElement.
 - Everything has a bounding box rect which is in absolute coordinates
   and is recalculated only when updateBoundingBox() is called.
 - Rename ScreenElement to PictureElement
 - Connection points are now regular point instances. Can be either
   Pixel or PercentPoints.
 - All points except AbsolutePoint are relative to their parent.
Merge branch 'master' of github.com:skogler/slothdiagram

Conflicts:
	slothdiagram/src/main/java/org/slothdiagram/DrawActivity.java
Merge branch 'master' of github.com:skogler/slothdiagram

Conflicts:
	slothdiagram-instrumentation-tests/src/main/java/org/slothdiagram/ui/GraphicsViewTest.java
	slothdiagram/src/main/java/org/slothdiagram/DrawActivity.java