Added cast method to Accessor
addAsset method

Handy when you have java object representing a ModelEntity (game.json or
a scene), and you want it to associate it to a file.
GameLoader sets the default language
GameAssets is only cleared if its loading path actually changes
EffectsComponent combine
Tags components combine
Listeners are notified also when a variable is registered
Constructor with custom EngineApplicaitonListener for EngineDesktop
Fixed Exporter Test
Impossible to compare strings, since serialization of objects
write fields in random order.
Merge pull request #649 from anserran/geography

Tiny bugfixes related to the Geography game development