Implement system for easily adding YUI elements that draw for GM game objects

[shdwcat]

Currently in order to render YUI stuff at the position of a GM object in world space, you have to do advanced things like binding xoffset and yoffset to the x and y of the object, and using yui_world_to_gui_x/y to translate the coordinates. It would be much more ideal if there was a way to describe which game objects should draw YUI elements, as well as how to position the YUI content relative to the object (since often we'll want to align to the object bounding box in some way, similar to popup placement)