implement currentParts() for every elementItemEditor so the editor can be extended to multi part edit in a later step

sources/editor/ui/rectangleeditor.cpp

@@ -103,6 +103,10 @@ CustomElementPart *RectangleEditor::currentPart() const {
 	return m_part;
 }
 
+QList<CustomElementPart*> RectangleEditor::currentParts() const {
+    return m_style->currentParts();
+}
+
 /**
  * @brief RectangleEditor::topLeft
  * @return The edited topLeft already mapped to part coordinate