Fixes #179 .

The first time a project was loaded, EditorIO would execute with scenes a ModelCommand, which would set Model's scenes reference equal to EditorIO scenes reference. This would cause that the second time an OpenGame would be executed, a ModelCommand would clear the previous scenes(which are the same as the current scenes because they are the same reference, ModelCommand.java line: 66), so the new Model would have 0 scenes.
Closes #179 .