unified draggable, rts, and physics objects

Engine Changes:
-added get_type() to classes that were missing it
-moved UI classes to UI namespace
-added default colors to UI classes
-removed draggable and rts objects
-removed save() and load() from physics_object
-completely removed overlay
-created new class "model" to replace draggable, rts, and physics
objects

Demo Changes:
-changed items in dropdown menu
-object selection and deleting is buggy