Add ro_mouse module to process mouse events during drags and on null polls.
Implement support in ro_mouse for tracking the mouse during drags and passing on DragEnd events.
Remove ro_gui_drag_end() and update all of its clients so that they use ro_mouse.
Remove the unused ro_gui_window_frame_resize_end().
Termination of save drags with Escape is unimplemented, but appears broken anyway.
The use of gui_drag_type is still required to handle Message_DatasaveAck processing.
Mouse tracking is still handled via gui.c.
