Moves star difficulty calculation entry-point to Beatmap, and sets star difficulty at the correct place for song select to display.
Note that this is a very naive approach and will result in file access exceptions. This will be fixed in a further commit.
They can now only be instantiated from BeatmapDatabase and are abstract (to avoid misuse).
Renames Colour to BackgroundColour instead.
Add loading screen.
Adds base classes for difficulty calculations, hooks them up with carousel container, and adds a port of the osu difficulty calculator.