Brush/Paint internal changes

Brush/Paint internal changes
- remove brush array for each Paint struct, just use a single brush pointer.
- removed rna function based template filtering.
- filter brushes using a flag on the brush and the pointer poll function.
- set the brushes using a new operator WM_OT_context_set_id().

TODO
- remake startup.blend, currently brush groupings are lost.
- rewrite WM_OT_context_set_id() to use rna introspection.