Changes to make services less dependant on each other.

These changes were made so that we can gradually move away from our
Engine super object and have available to use only the things we need.

* Added GetInstance functions for Engine, UnitService and RestingService
classes. GetInstance returns a singleton object for the given class.
* Changed all references to Config's FilterInfo field into UnitService's
new FilterInfo field.
* RestingService and UnitService now only depend on FFACE
implementations.
* Session class is now renamed FFInstance again.