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/SketchObject.cpp

@@ -234,7 +234,10 @@ int SketchObject::setDriving(int ConstrId, bool isdriving)
         type != Radius &&
         type != Angle &&
         type != SnellsLaw)
-        return -1;
+        return -2;
+    
+    if (!(vals[ConstrId]->First>=0 || vals[ConstrId]->Second>=0 || vals[ConstrId]->Third>=0) && isdriving==true)
+        return -3; // a constraint that does not have at least one element as not-external-geometry can never be driving.
 
     // copy the list
     std::vector<Constraint *> newVals(vals);
@@ -245,11 +248,7 @@ int SketchObject::setDriving(int ConstrId, bool isdriving)
     this->Constraints.setValues(newVals);
     delete constNew;
 
-    int err = solve();
-    if (err)
-        this->Constraints.setValues(vals);
-
-    return err;
+    return 0;
 }
 
 int SketchObject::getDriving(int ConstrId, bool &isdriving)
@@ -273,6 +272,38 @@ int SketchObject::getDriving(int ConstrId, bool &isdriving)
     return 0;
 }
 
+int SketchObject::toggleDriving(int ConstrId)
+{
+    const std::vector<Constraint *> &vals = this->Constraints.getValues();
+    
+    if (ConstrId < 0 || ConstrId >= int(vals.size()))
+        return -1;
+    
+    ConstraintType type = vals[ConstrId]->Type;
+    
+    if (type != Distance &&
+        type != DistanceX &&
+        type != DistanceY &&
+        type != Radius &&
+        type != Angle &&
+        type != SnellsLaw)
+        return -2;
+    
+    if (!(vals[ConstrId]->First>=0 || vals[ConstrId]->Second>=0 || vals[ConstrId]->Third>=0) && vals[ConstrId]->isDriving==false)
+        return -3; // a constraint that does not have at least one element as not-external-geometry can never be driving.
+        
+    // copy the list
+    std::vector<Constraint *> newVals(vals);
+    // clone the changed Constraint
+    Constraint *constNew = vals[ConstrId]->clone();
+    constNew->isDriving = !constNew->isDriving;
+    newVals[ConstrId] = constNew;
+    this->Constraints.setValues(newVals);
+    delete constNew;
+
+    return 0;
+}
+
 int SketchObject::movePoint(int GeoId, PointPos PosId, const Base::Vector3d& toPoint, bool relative)
 {
     Sketch sketch;
@@ -489,7 +520,7 @@ int SketchObject::setConstruction(int GeoId, bool on)
     newVals[GeoId]=geoNew;
 
     this->Geometry.setValues(newVals);
-    this->Constraints.acceptGeometry(getCompleteGeometry());
+    //this->Constraints.acceptGeometry(getCompleteGeometry()); <= This is not necessary for a toggle. Reducing redundant solving. Abdullah
     return 0;
 }