Game selection support, export code into PyDoom.zip, more

- Added a game selector to choose which game to load, and also some quick
  placeholder definition modules for soon-to-be-supported games.
- Exported the static Python code into PyDoom.zip's __main__ module.
- We now search PyDoom.zip for the main program's code. The program won't run if
  PyDoom.zip is missing, but this is fine since it's going to cause a million
  problems if it's not there anyway.
- PyDoom_OpenGL is now internal, and is instead accessible by the module name
  'pydoom.opengl'.