TID: Allocate internal IDs to mobjs

[LoboEire]

Allocate internal IDs to mobjs.
That always increment and are thus never repeated during a level.
Then clear out the ID list when changing levels.
This way, specific mobj can be searched for and returned to LUA/Coal without having to rely on TAGs.

The lookup could probably be something like std::unordered_map<uint64_t, MapObject *> mobj_list
And then you would do mobj_list[TID]