Split up FNAPlatform.RunLoop

[flibitijibibo]

This one's mostly for @TheSpydog who's currently exploring wasm, but also applies to @jfmajor who needs this feature for their editor.

Currently the loop is held entirely in the FNAPlatform function, but this is a problem for 2 reasons:

1. RunOneFrame needs to poll events before Tick(), to handle certain updates (key up/down, hotplugging, text input, things like that)
2. Every FNAPlatform conceivable is going to have a loop anyway, and a lot of code at the top and bottom of RunLoop could easily be shared

This is a proposed refactor, but could change depending on performance/accuracy issues we've not encountered yet:

• FNAPlatform.RegisterGame shows the game window and adds it to activeGames
• FNAPlatform.UnregisterGame removes the game from activeGames
• FNAPlatform.PollEvents contains the SDL_Event loop, add this to RunOneFrame()
• OSXUseSpaces becomes a static readonly variable
• List<Keys> keys is initialized in Keyboard.cs instead
• Text input variables become Game instance variables
• displayIndex becomes a GraphicsAdapter rather than an int
• Game would get a new RunLoop that looks like this:

private void RunLoop()
{
    while (RunApplication)
    {
        FNAPlatform.PollEvents(
            this,
            currentAdapter,
            textInputControlDown,
            textInputControlRepeat,
            ref textInputSuppress
        );
        Tick();
    }
    Exit();
}

protected virtual void BeginRun()
{
    if (loopBegan)
    {
        return;
    }

    currentAdapter = FNAPlatform.RegisterGame(this);

    Rectangle windowBounds = Window.ClientBounds;
    Mouse.INTERNAL_WindowWidth = windowBounds.Width;
    Mouse.INTERNAL_WindowHeight = windowBounds.Height;

    // Perform initial check for a touch device
    TouchPanel.TouchDeviceExists = GetTouchCapabilities().IsConnected;

    loopBegan = true;
}

protected virtual void EndRun()
{
    if (loopBegan)
    {
        FNAPlatform.UnregisterGame(this);
        loopBegan = false;
    }
}

[flibitijibibo]

Fixed by #324