Notion of edition context added to model
Updated selectioncommand to set contexts
Updated set selection action.
Added set edition context action
scene listener uses setEditionContext action
AddSceneElement uses context as parent of the added entity
Edit scene updates the edition context
Added GroupEditorConfiguration to group editor widgets.
Added longPress event in MockupSceneEditor.
Move WalkComponent to components
Merge pull request #539 from anserran/contexts

Added notion of edition contexts
Merge pull request #544 from manuel-freire/move-walkcomponent

Move WalkComponent to components
Cleaner icons
Small Scenes bugfix.
Improved EditorGameAssets#resolve(String) method
EditorGameAssets now loads engine skin.
Remove static reference in CommandTest
Reduced the size of buttons and added new icon.
Added panel in scene edition with next options:
	Delete selected elements
	Move to back or front the selected elements
	Go to edit element
- addComponent moved from EntitiesLoader to ComponentLoader and made public
- AddComponentExecutor now uses ComponentLoader.addComponent instead of creating components by itself