[TD] handle object deletions better

see https://forum.freecadweb.org/viewtopic.php?p=366902#p366902

src/Mod/TechDraw/Gui/ViewProviderDimension.cpp

@@ -221,3 +221,12 @@ void ViewProviderDimension::handleChangedPropertyType(Base::XMLReader &reader, c
         LineWidth.setValue(LineWidthProperty.getValue());
     }
 }
+
+bool ViewProviderDimension::canDelete(App::DocumentObject *obj) const
+{
+    // deletions of objects from a ProjGroupItem don't necesarily destroy anything
+    // thus we can pass this action
+    // we can warn the user if necessary in the object's ViewProvider in the onDelete() function
+    Q_UNUSED(obj)
+        return true;
+}