"Filter tree" in Outline should be a simple button instead of a drop-down menu for QML files

1. Open a QML file.
2. Show the Outline in the sidebar.
3. Click the "Filter tree" icon.
   You'll see a drop-down menu which contains just one item:
   

Having a button to open a single item drop-down menu is pointless. Instead, there should be a button triggering the action which is now being triggered by the menu.