Added layer ids to a separate class so it can be easily referenced from GameLayers but also from outside
GameLayers refactored:
- Flexible layer-structure independent design. We can change how layers are arranged by adding/removing/editing Layer class
- Added a method for clearing layers but preserving layer structure
- Methods renamed to sth that suits them better
Simple test for layers
EntitiesLoader refactored:
- GameLayers has been taken out from EntitiesLoader. Now EntitiesLoader just loads the elements, but don't get them actually added to the view.
- Public methods have been renamed to a better choice. Now we have two methods: "loadEntity" and "toEngineEntity".
- Javadoc improved
GameLoader and GoSceneExecutor refactored after latest changes in the API
Other refactorings
Better handling of shortcuts
Refactor associated to changing ShortcutsMap
Added desktop shortcuts
Bugfix in ViewsHistory
Updated Clipboard to use selection instead of asking views
EditorGameAssets sets components ignores

Clipbard also needs to serialize objects with editorGameAssets, so Save
can't set the ignores no more.
Updated Copy, Cut and Paste actions
Created ModelEntityCopyListener to copy/cut/paste model entities
Changes after refactor
Comments addressed
Merge pull request #513 from jtorrente/gamelayers

Tidying up game layers and entities loader
Merge pull request #519 from anserran/shortcuts

Better handling of shortcuts
Merge pull request #520 from anserran/viewshistorybug

Bugfix in ViewsHistory
Merge pull request #523 from anserran/copypaste

Updated Copy-Paste mechanism to use selection