Whitespace tweaks and changed position of signal declarations

The signal declarations were positioned after the private variables in
Box2DFixture and Box2DChain, which is uncommon.
Don't have DebugDraw darken the view automatically

Drawing this large transparent rectangle is a major performance issue
for the QQuickPaintedItem on large screens. When it is desired, it's
easy and much faster to do it with a transparent rectangle in QML.
Removed the need to keep a list of bodies in Box2DWorld

Box2D itself is already has the list of bodies so there is no need for
another list. When the world is deleted, it is enough to make sure the
Box2DBody instances no longer reference the deleted b2Body instances.