Use make_intrusive/make_shared for smart pointer construction.

src/gui/symbol/symmetry_editor.cpp

@@ -118,7 +118,7 @@ void SymbolSymmetryEditor::onCommand(int id) {
     }
     resetActions();
   } else if (id == ID_ADD_SYMMETRY) {
-    symmetry = intrusive(new SymbolSymmetry());
+    symmetry = make_intrusive<SymbolSymmetry>();
     symmetry->kind   = SYMMETRY_ROTATION;
     symmetry->copies = 2;
     symmetry->center = Vector2D(0.5,0.5);