Ui improvement

Fixes the rotation of the GraphicalElements displayed in the CompositionArea,
Fixes the align action to align the CompositionJPanel and not the GraphicalElement,
Fixes the selection of an ows-context,
The GraphicalElement follows the mouse when it's dragged,
The images loaded in different GraphicalElement are scaled to the GrapphicalElement size and not cut

Now there are three ways to resize with the mouse a GraphicalElement : 
* By resizing with the mouse : resize directly the GraphicalElement. Fast and useful when the GE rotation angle is 0°. But useless when the angle is different from 0.
* By holding the AltGr key and resizing with the mouse : resize the representation (the CompositionJPanel) corresponding to the GE. The CompositionJPanel will take the new dimension and the GraphicalElement will be adapted to fit inside the rectangle. But still some minor bug to fix (Sometimes 1 pixels too wide or too short)
* By holding the Shift key and resizing with the mouse, the GraphicalElement will keep the ratio width/height.