pedantic word ordering change.

- wm.add_modal_handler -> modal_handler_add
- wm.add_fileselect -> fileselect_add
- ob.add_shape_key -> shape_key_add
- VIEW3D_OT_add_background_image -> VIEW3D_OT_background_image_add (same for remove)

Also made 2 internal cmake vars hidden.

release/scripts/op/wm.py

@@ -506,7 +506,7 @@ class WM_OT_context_modal_mouse(bpy.types.Operator):
         else:
             self.initial_x = event.mouse_x
 
-            context.window_manager.add_modal_handler(self)
+            context.window_manager.modal_handler_add(self)
             return {'RUNNING_MODAL'}