Logic needed for mapping ModelComponents to ClassComponents has been moved from RemoveComponentExecutor to EntitiesLoader, since it is going to be reused.
Also, a method EntitiesLoader.getClass(String alias) has been added in EntitiesLoader so there is no need to pass an EntitiesLoader and a GameAssets to RemoveComponentExecutor
Several improvements to Accessor so it can be used to access entity properties in the engine:
- Added support for Array, IntMap, ObjectMap
- Added support for different types of keys in maps.
- Added support for auto-mapping model components into engine components so things like "entity.components<visibility>" can be parsed and evaluated in expressions

Also: Test updated (it was commented out)
Added a "prop" operator that allows accessing entities' properties in expressions.
To see more details on how it works, see the hunk in ParserTest
Does not need a lot of attention.
Reflecting changes in API:
- VariablesSystem and OperatorFactory now require EntityLoader argument
Comments addressed + javadoc improved + other minor fixes
Merge pull request #414 from jtorrente/property_accessor

Property operator