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
Adding an easy way to iterate through entities. Fixes #525
EngineLayers are added to the engine. Test updated
GameLoader updated to prevent layers getting removed unexpectedly
minor fix
Merge pull request #550 from jtorrente/addlayerstoentity

Layers added to engine
Added new model action
Fixed some code and a reorder bug (There were empty rectangles after ordering)
Merge pull request #551 from RotaruDan/elementSelectedWidget

Element selected widget