Sketcher: Bug fix External geometry not appearing upon insertion

================================================================

A last moment change to regulate redrawing introduced this issue (reducing the redrawing to the cases where the solver geometry and the sketchObject geometry are equal).
This change also solved other issues.

The case of adding external geometry is special because the DoF is the same after insertion, yet there is an insertion (as opposed to the toggle case, where there is a change but no insertion).

SketchObject has been adapted to require a solver update before moving (which also caused a non-yet-reported crash, a separate bug), and CommandCreateGeo has been adapted to trigger a solve with
auto update mode disabled. This solve is a very low cost one as the geometry is already in place.

src/Mod/Sketcher/App/SketchObject.cpp

@@ -1887,6 +1887,7 @@ int SketchObject::addExternal(App::DocumentObject *Obj, const char* SubName)
         return -1;
     }
 
+    solverNeedsUpdate=true;
     Constraints.acceptGeometry(getCompleteGeometry());
     rebuildVertexIndex();
     return ExternalGeometry.getValues().size()-1;
@@ -1938,7 +1939,8 @@ int SketchObject::delExternal(int ExtGeoId)
         ExternalGeometry.setValues(originalObjects,originalSubElements);
         return -1;
     }
-
+
+    solverNeedsUpdate=true;
     Constraints.setValues(newConstraints);
     Constraints.acceptGeometry(getCompleteGeometry());
     rebuildVertexIndex();

src/Mod/Sketcher/Gui/CommandCreateGeo.cpp

@@ -4545,6 +4545,14 @@ class DrawSketchHandlerExternal: public DrawSketchHandler
 
                     if(autoRecompute)
                         Gui::Command::updateActive();
+                    else {
+                        // adding external geometry does not require a solve() per se (the DoF is the same), 
+                        // however a solve is required to update the amount of solver geometry, because we only
+                        // redraw a changed Sketch if the solver geometry amount is the same as the SkethObject
+                        // geometry amount (as this avoids other issues).
+                        // This solver is a very low cost one anyway (there is actually nothing to solve).
+                        static_cast<Sketcher::SketchObject *>(sketchgui->getObject())->solve();
+                    }
 
                     Gui::Selection().clearSelection();
                 /* this is ok not to call to purgeHandler