Add support for old beatmaps
Note, that circle sizes still are wrong compared to stable osu. In order to fix this, the base radius of hitcircles needs to become 64, but it currently is 72.
Slider & other hit object improvements.
General fixes
No big optimizations yet, but groundwork is laid out.
# Conflicts: # osu.Game.Mode.Osu/UI/OsuComboCounter.cs # osu.Game.Mode.Osu/UI/OsuScoreOverlay.cs
# Conflicts: # osu-framework # osu.Game/GameModes/OsuGameMode.cs # osu.Game/GameModes/Play/Player.cs # osu.Game/OsuGame.cs # osu.Game/Overlays/MusicController.cs # osu.Game/Overlays/Options/EditorSection.cs # osu.Game/Overlays/Options/Input/MouseOptions.cs # osu.Game/Overlays/Options/Online/InGameChatOptions.cs # osu.Game/Overlays/Options/SkinSection.cs