Window classes and alternate backends

Originally osu!framework used the OpenTK library for windowing, then transitioning to a custom fork of it (osuTK), and finally moving over to SDL2. The transition is not fully complete; the only platform remaining which is dependent on osuTK is Android.

The class hierarchy currently looks as follows:

classDiagram
    direction LR
    class IWindow
    <<interface>> IWindow
    IWindow <|-- OsuTKWindow
    OsuTKWindow <|-- AndroidGameWindow
    IWindow <|-- SDL2Window
    SDL2Window <|-- IOSWindow
    SDL2Window <|-- SDL2DesktopWindow
    SDL2DesktopWindow <|-- MacOSWindow
    SDL2DesktopWindow <|-- WindowsWindow

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• IOSWindow contains iOS-specific SDL calls to enable CADisplayLink to work for lower latency.
• MacOSWindow contains platform-specific code for precise scroll wheel handling.
• WindowsWindow contains platform-specific code for DPI awareness, IME, icon setting, and raw input handling.