Blender Git Loki

2.5: Data Context

* Worked out data context implementation more, now with initial context
callbacks implemented for the screen and 3d view.
* For collections, switch from iterators to simpler ListBase. Though that still
means it uses LinkData* rather than the actual Object* for example, since
those can only be part of one list. So I added a macro as well to make
iteration easier when possible.

CTX_DATA_BEGIN(C, Object*, ob, selected_objects) {
printf("object name: %sn", ob->id.name);
}
CTX_DATA_END;