Sketcher: Driving/reference creation improvements and some other fixes

======================================================================

- Changing from Driving to reference does not include unnecessary solvings.
- Added some checks to avoid making Driving constraints when calling directly from python and involving only external geometry (would give redundant constraints).
- New python command toggleDriving to just change the status from reference to Driving
- New UI toolbar Command to toggle constraints
- Fix to allow switching from/to construction mode during continuous mode creation.
- Enable/Disable for constraints in constraints widget has changed to operate on multiselection and now effects "toggle" instead of enable/disable.
- Disable the option to directly create a SnellsLaw non-driving constraint (this constraint does not support direct creation, it can be toggled to non-driving after creation though).

src/Mod/Sketcher/App/SketchObjectPy.xml

@@ -87,6 +87,11 @@
       <Documentation>
         <UserDocu>Get the Driving status of a datum constraint</UserDocu>
       </Documentation>
+    </Methode>
+    <Methode Name="toggleDriving">
+      <Documentation>
+        <UserDocu>toggle the Driving status of a datum constraint</UserDocu>
+      </Documentation>
     </Methode>    
     <Methode Name="movePoint">
       <Documentation>