Added a simple "LOADING" screen when ES starts up.
ViewController now preloads GameListViews so there's no lag when browsing
to a system for the first time.
If you know something is about to cause the framerate to tank (e.g. theme
loading), you can tell the next update to be, at maximum, the average of
the previous five seconds of frames.
The framerate drawing code has also been moved to Window.
The ResourceManager provides a unified interface for accessing resource
data, embedded or from the filesystem, with
initialization/deinitialization handled automatically behind the scenes.
It also keeps from creating duplicate resources (e.g. when two
ImageComponents use the same image file).
Audio still needs to be moved over to it.
SDL_mixer is not in the standard SDL distribution. The mixing is now
done using regular SDL_Audio functions. AudioManager is converted to a
singleton and std::shared_ptrs are used for all Sound objects. Note that
for GCC "-std=c++11" might need to be added to the CMAKE_CXX_FLAGS.
