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

This PR contains several modifications over the Model:
Two commits to move all save-related logic to the Save action.
https://github.com/jtorrente/ead/commit/fee1e6e30fc40bc066c3bbdabdbf7f15754f813d
https://github.com/jtorrente/ead/commit/f1638d0729c844c7582c0c9f0ba4dc28c0acd260

Two commits to move all load-related logic to the OpenGame action:
https://github.com/jtorrente/ead/commit/043681feed93309714c4c9c3a42d75e29e38ad65
https://github.com/jtorrente/ead/commit/3533f7681dfd3aa290a96afb376dffa8b91e94bf

One more commit to remove editorGameAssets from the model, as it is not needed anymore
https://github.com/jtorrente/ead/commit/d10906c7f3b2868a1a3e10474b6f80bc1344d346

Three commits for adding support in the model for getting interactive elements based on their type. A test is missing, but I'll add it in a separate PR that fixes tests (once this PR is approved).
https://github.com/jtorrente/ead/commit/32acbedc4d9e48417abf2c1a9b11977317a7b7fd
https://github.com/jtorrente/ead/commit/1cfe5afc4a068a1b36f45452107477427da7cad6
https://github.com/jtorrente/ead/commit/a9de65b7dbdfc60c5e93aac49850797e79079bc9

Finally, a little refactoring in Save to make use of the recenlty created ModelEntityFilter
https://github.com/jtorrente/ead/commit/58f998fa805dd70e5d4ec5cd5008a841b8479f3b