Decicion modeling plugin: tooltips refactoring

[Chabrier]

In order to improve the tool tip management of the drawing area of the decision plugin some refactoring has to be done.

• The method to use is OnQueryToolTip
• The information getter(the information we want to display) should be of a modeling class, in the sens of MVC

The class to change is : the DecisionDrawingArea The class to take as example id ViewDrawinArea of gvle.

The behavior of the GUI should stay exactly the same.