Resolves the issue #13,
Creates an UILayer for the CompositionArea. This layer will be used as Overlay to permit the user to specify the new GraphicalElement bounding box with the mouse. In the future it will be also used to display messages.
Now when a new GraphicalElement will be created, the UIController save the class of the GE, and ask the CompositionArea to enable the CompositionAreaOverlay. The overlay will get the GE bounding box, return it to the UIController that will create the GraphicalElement.
Adds in the UIController a GE List (toBeSet) to allow the user to configure a particulare GraphicalElement without loosing the liste of GE selected.
Corrects the readOnly state of the StringCA (by default it was at 'true').