Support for implementing package-external handlers

[skalarproduktraum]

This commit introduces a few getters and makes some properties accessible, such that external handlers (apart from Swing) may be implemented, such as JOGL or LWJGL. In detail:

• BehaviourMap and InputTriggerMap gain a modCount() getter
• the _MASK static properties of InputTrigger are now public static

[tpietzsch]

thanks @skalarproduktraum.