Rename callable_method_pointer.h to callable_mp.h and include it explicitly

This allows removing it from `class_db.h`, significantly reducing the amount of files
that include it transitively.

Also includes some include cleanup in `control.h` and `rich_text_label.h` done while
ensure they don't depend on `callable_mp`.

scene/gui/split_container.cpp

@@ -32,6 +32,7 @@
 #include "split_container.compat.inc"
 
 #include "core/config/engine.h"
+#include "core/object/callable_mp.h"
 #include "core/object/class_db.h"
 #include "scene/gui/texture_rect.h"
 #include "scene/main/viewport.h"