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/SketchObjectPyImp.cpp

@@ -49,7 +49,7 @@ using namespace Sketcher;
 
 // returns a string which represents the object e.g. when printed in python
 std::string SketchObjectPy::representation(void) const
-{
+{        
     return "<Sketcher::SketchObject>";
 }
 
@@ -578,7 +578,7 @@ PyObject* SketchObjectPy::setDriving(PyObject *args)
 
     if (this->getSketchObjectPtr()->setDriving(constrid, PyObject_IsTrue(driving) ? true : false)) {
         std::stringstream str;
-        str << "Not able set Driving for constraint with the given index: " << constrid;
+        str << "Not able set Driving/reference for constraint with the given index: " << constrid;
         PyErr_SetString(PyExc_ValueError, str.str().c_str());
         return 0;
     }
@@ -603,6 +603,23 @@ PyObject* SketchObjectPy::getDriving(PyObject *args)
     return Py::new_reference_to(Py::Boolean(driving));
 }
 
+PyObject* SketchObjectPy::toggleDriving(PyObject *args)
+{
+    int constrid;
+    
+    if (!PyArg_ParseTuple(args, "i", &constrid))
+        return 0;
+
+    if (this->getSketchObjectPtr()->toggleDriving(constrid)) {
+        std::stringstream str;
+        str << "Not able toggle Driving for constraint with the given index: " << constrid;
+        PyErr_SetString(PyExc_ValueError, str.str().c_str());
+        return 0;
+    }
+
+    Py_Return;
+}
+
 
 PyObject* SketchObjectPy::movePoint(PyObject *args)
 {