Adding the StemModel (for diracs); improving the onPick() method in XYView; reducing...

Adding the StemModel (for diracs); improving the onPick() method in XYView; reducing the number of repaint() called when setCurrentCurve() is called; Adding the opacity parameter in settings; adding a flatten() function in utils to ease the use of the same mplArtist member for all Models; Adding an hline marker in the list of markers; Adding actions in the right-click menu to show all traces or only the selected one; Clickable legend is fully functionnable (text and symbol handles)