PcDisplay: now handles display.mode.
Remove old resources.path code from GameEngine.
PcDisplay now has a display.fps setting.

This controls whether the RateCounter is enabled or not.
ResourceManager now uses Settings.OnChangedListener.
AbstractSettingsTest now tests Settings.OnChangedListener.
Create a NotificationManager.

This will be uesful to avoid duplication between OnChangedListener,
OnKey, etc in codebase.
AbstractInputManager now uses the NotificationManager.