Fixes arround Graph Panel

 - Fix 692 - Crash Opening Graph Panel …  http://www.synfig.org/issues/thebuggenie/synfig/issues/692
 - Graph Panel : Fix curves line witdh
 - Graph Panel : Fix scrolling
 - Fix 693 - Graph panel, mouse wheel should just scroll ...  www.synfig.org/issues/thebuggenie/synfig/issues/693

++ Fix Panel's context menu ([x] close) http://www.synfig.org/issues/thebuggenie/synfig/issues/708