Blender Git Loki

WIP redesign of the system. Nothing functional (not even manipulator).

There is first code for display of an arrow widget. The manipulator will
use those when they are ready.

Basically the manipulator is now a WidgetGroup. WidgetGroups are a new entity and
they manage a group of related widgets. They should be responsible for state update,
drawing and polling of their widgets and can be bound to specific rules.

There is a big design plan on a whiteboard beside me, it will take a few
big commits still to take shape though. One thing at a time, I am first making
reusable arrow widgets that will allow us replace the translate widgets properly.

There are a few reusable widgets planned, more to come later.