* The new NAVMESH Modifier did not show any buttons, console printed errors instead!
* Poll of "PHYSICS_PT_game_obstacles" panel caused errors as well, self instead of cls was used as argument.
* Check to show/hide buttons in "WORLD_PT_game_physics_obstacles" panel did not worked due to wrong ENUM identifier ('None' instead if 'NONE')
* Moved "SCENE_PT_navmesh" panel out of properties_scene.py into the properties_game.py where it belongs and renamed it. Also, don't use abreviations in Panel Headers (Navmesh > Navigaion Mesh)
* Code cleanup, removed unnescecary code.
* bpy.types.Panel > Panel
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