how to use this framework:

1. Midlet:
	the derived midlet class has to override the method: createxApplicationInstance();
2. Application:
	should create a class that derived from xApplication, and should override onStartApp, onDestroyApp
	to create the screens, call setScreen to set the current screen
3. Screen:
	all screens have to extend from this class, should override:
		onFocus, onLostFocus
		onTick, render
		onKeyDown, onKeyUp
		onSuspend, onResume
	

xCanvas: this class should be modified depend on the specified platform