Sub out a concept of Subsystems in :core.
Add missing @Override's to last commit.
Fix compiler errors in :core.
:pc now stubbed for Subsystem.
:android:common now stubbed for Subsystem.
ResourceManager now functions as a Subsystem.
ResourceManager's default loader is now static.
Freshen ResourceManager's JavaDoc comments.
ResourceManager: note that resources.path is handled here.
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.
Initialize Subsystems before mRuntimeSettings setup.

This will make the Subsystems handle their runtime
settings before we call start().
Create a Settings.OnChangedListener helper class.
ResourceManager now uses SettingsListener.
PcDisplay now uses SettingsListener.