Model tweaking: (1) Save, (2) Load, (3) Entity categories

This PR contains several modifications and refactoring over the Model, how it is loaded, saved, accessed and modified.
Two commits to move all save-related logic to the Save action.
https://github.com/jtorrente/ead/commit/68663c148bbf812476c76db3580468231dee18fe
https://github.com/jtorrente/ead/commit/953e5ea95d4ad705beba6f11c9722d63c49e8d18

Two commits to move all load-related logic to the OpenGame action:
https://github.com/jtorrente/ead/commit/9f751ae427e7b329a39c9b34dfd43316ea3370cf
https://github.com/jtorrente/ead/commit/054b93d532f73c9b494453d2322df4560cd0f809

One more commit to remove editorGameAssets from the model, as it is not needed anymore
https://github.com/jtorrente/ead/commit/cb337ee7b2d2d1e44bbfa2ff1ecf7b4909c5108b

Three commits for adding the concept of ModelEntityCategories in the schema and for simplifying, and cleaning out the Model.
https://github.com/jtorrente/ead/commit/e62fe7bdbd90cee3c58c10f19c8d0d642058b5c0 (this commit makes sense after the next one)
https://github.com/jtorrente/ead/commit/9f8fedb05ee46fbbbe06623fa86c05d665f92a96
https://github.com/jtorrente/ead/commit/af35b3642ebccc6a2b84953efc4b7cb5357bb48a

Three more commits to simplify actions that iterate through ModelEntities, like Save, Open, New, Export, thanks to the new model structure. Also, more revising on how games are loaded.
https://github.com/jtorrente/ead/commit/d327a41e32261ebe734a408ac024b5f638727a40
https://github.com/jtorrente/ead/commit/c9084b224a55ef45d204dc379731765212501c10
https://github.com/jtorrente/ead/commit/7bbc5b7734cd8532cdf68c8225123a19690f4edb

Finally, one more commit to replace occurrences of old Model.getScene() method
https://github.com/jtorrente/ead/commit/117b8b40e493e870919a03e00342ffbf5d05f353