Sketcher: Add FullyConstrained read-only output property

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This property stores whether a Sketch has 0 DoF or not upon solve.

It is serialised to disk, enabling a recently loaded project to have an accurate
populated property even before the first solve() call.

src/Mod/Sketcher/App/SketchObject.cpp

@@ -110,6 +110,7 @@ SketchObject::SketchObject()
     ADD_PROPERTY_TYPE(Geometry,        (0)  ,"Sketch",(App::PropertyType)(App::Prop_None),"Sketch geometry");
     ADD_PROPERTY_TYPE(Constraints,     (0)  ,"Sketch",(App::PropertyType)(App::Prop_None),"Sketch constraints");
     ADD_PROPERTY_TYPE(ExternalGeometry,(0,0),"Sketch",(App::PropertyType)(App::Prop_None),"Sketch external geometry");
+    ADD_PROPERTY_TYPE(FullyConstrained, (false),"Sketch",(App::PropertyType)(App::Prop_Output|App::Prop_ReadOnly |App::Prop_Hidden),"Sketch is fully constrained");
 
     Geometry.setOrderRelevant(true);
 
@@ -251,6 +252,7 @@ int SketchObject::solve(bool updateGeoAfterSolving/*=true*/)
     lastDoF = solvedSketch.setUpSketch(getCompleteGeometry(), Constraints.getValues(),
                                   getExternalGeometryCount());
 
+    FullyConstrained.setValue(lastDoF == 0);
     // At this point we have the solver information about conflicting/redundant/over-constrained, but the sketch is NOT solved.
     // Some examples:
     // Redundant: a vertical line, a horizontal line and an angle constraint of 90 degrees between the two lines

src/Mod/Sketcher/App/SketchObject.h

@@ -67,6 +67,7 @@ public:
     Part    ::PropertyGeometryList   Geometry;
     Sketcher::PropertyConstraintList Constraints;
     App     ::PropertyLinkSubList    ExternalGeometry;
+    App     ::PropertyBool           FullyConstrained;
     /** @name methods override Feature */
     //@{
     short mustExecute() const override;