Resolve issue #13,
On clicking on a button to create a GE, the user can specify its bounding box by pressing, dragging and releasing the mouse in the CompositionArea,
In the compositionArea, to get the mouse positions and draw the box, a transparent JPanel is added at the front and sent the data to the UIController,
Splits the creation of a new GE in the UIController to make it able to create one with the given size or only with the given class,
Add to the GEFactory a method to create a GE giving its size.