Utility & convenience functions, Javadoc fixes

* finishRendering() is called automatically by the engine now. No need
to do it manually. (I don't even know why it was that way before)
* The ShapeRenderer can draw a polyline using a Polygon holding the
data.
* Methods added in Mouse class to test whether any [of the given]
buttons is pressed (-> anyDown()).
* Level.getEntities() can be given null, in which case a new list is
allocated and returned. Also, Level.getEntityCount() and
Level.getEntity(int index) were added.
* The Level class now provides a function for converting screen
coordinates into level coordinates.
* It is now possible to override a function in TiledLevel called
renderTile to do additional/other rendering for tiles.

* The Javadoc was updated again. This addresses errors & warnings spit
out by the javadoc command. I must have missed a few things last time.