VariablesSystem renamed to VariablesManager
VariablesManager is not a System anymore.
Also: some outdated javadoc fixed along the way
VariablesManager added as a field of EngineApplicationListener that is passed down to GameLoader, EngineInitializer, etc.
Accommodating API changes (review not needed)
Package ...engine.systems.variables moved to ...engine.variables (review not needed)
Fixes #204
Model now uses complete paths as ids. Also, some convenient methods
Fixed classes after changes
Fixed tests
Added javadoc to GameLayers
Merge pull request #440 from manuel-freire/game-layers-javadoc

Added javadoc to GameLayers
Merge pull request #432 from anserran/fix204

Fixes #204
Accessor embedded in VariablesManager
Merge pull request #438 from anserran/modelid

Model now uses completes paths as ids
Merge pull request #437 from jtorrente/variablesManager

Variables manager - Fixes #394
Using plain objects for selections instead of Model entities