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/CommandCreateGeo.cpp

@@ -5350,7 +5350,16 @@ void CmdSketcherGeometryCreationMode::activated(int iMsg)
 
 bool CmdSketcherGeometryCreationMode::isActive(void)
 {
-    return isCreateGeoActive(getActiveGuiDocument());
+   Gui::Document * doc=getActiveGuiDocument();
+   
+   if (doc) {
+        // checks if a Sketch Viewprovider is in Edit and is in no special mode
+        if (doc->getInEdit() && doc->getInEdit()->isDerivedFrom
+            (SketcherGui::ViewProviderSketch::getClassTypeId())) {
+                return true;
+        }
+    }
+    return false;
 }
 
 void CreateSketcherCommandsCreateGeo(void)