Change region drawing callbacks to work much closer to how blender manages them internally.

- yes, this does break scripts, but the api is marked experimental.


ED_region_draw_cb_activate() adds a callback to a region type whereas the api made it look like the callback was being added to the region instance.
Use a class method on bpy.types.Space to manage region drawing, eg.

was:
  self._handle = context.region.callback_add(draw_callback_px, args, 'POST_PIXEL')

is now:
  self._handle = bpy.types.SpaceView3D.draw_handler_add(draw_callback_px, args, 'WINDOW', 'POST_PIXEL')

source/blender/python/intern/bpy_rna_callback.h

@@ -28,5 +28,10 @@
 struct BPy_StructRNA;
 struct PyObject;
 
+#if 0
 PyObject *pyrna_callback_add(BPy_StructRNA *self, PyObject *args);
 PyObject *pyrna_callback_remove(BPy_StructRNA *self, PyObject *args);
+#endif
+
+PyObject *pyrna_callback_classmethod_add(PyObject *cls, PyObject *args);
+PyObject *pyrna_callback_classmethod_remove(PyObject *cls, PyObject *args);