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/Gui/Workbench.cpp

@@ -206,6 +206,7 @@ inline void SketcherAddWorkbenchConstraints<Gui::MenuItem>(Gui::MenuItem& cons){
             << "Sketcher_ConstrainSnellsLaw"
             << "Sketcher_ConstrainInternalAlignment"
             << "Separator"
+            << "Sketcher_ToggleDrivingConstraint"
             << "Sketcher_ConstraintCreationMode";
 }
 
@@ -229,6 +230,7 @@ inline void SketcherAddWorkbenchConstraints<Gui::ToolBarItem>(Gui::ToolBarItem&
             << "Sketcher_ConstrainAngle"
             << "Sketcher_ConstrainSnellsLaw"
             << "Separator"
+            << "Sketcher_ToggleDrivingConstraint"            
             << "Sketcher_ConstraintCreationMode";            
 }