Hack P_MobjThinker into little pieces

Another 3000-line behemoth that I split into lots of subfunctions. Nothing should have changed functionality-wise or even in terms of execution flow. It's the same code but in smaller, more readable chunks.

This one could probably use some more thorough testing, because it affects such a huge chunk of the gamelogic.

srb2win-mobjthinker-refactor.exe

Strangely, this actually seems to improve performance. Several people (including myself) have reported significantly improved framerate in CEZ2 with this exe. If you're testing it, please report back how CEZ2's framerate changes for you.