Major rearanging/renaming of core components.

WARNING: This build is very unfinished and will NOT compile!

* ExObject merged into Entity
* Entity renamed to TmxObject
* Level renamed to TmxMap
* Several components of TmxObject moved into a new helper class called
  TmxMapData
* Screen renamed to Boundary
* NamedGroup renamed to TmxGroup; now uses strictly TmxObject as content
  and does not allow other types.
* BaseLevelLoader renamed to TmxMapLoader
* TmxMapLoader is much more modular now and no longer needs to be
  subclassed (or at least that's the plan)
* Many other changes that I can't remember