[REG Qt 5.5.1 -> Qt 5.6.0] Arrows in "Projects" view change to plus signs after using Quick Designer

1. Run a Creator based on Qt 5.6.0.
2. Open a project.
3. Show the "Projects" view.
   The view shows small arrow icons to expand its nodes:
   
4. Open a qml file in Quick Designer.
5. Show the "Projects" view again.
   It now shows plus signs instead of the arrows:
   
   These still work in the same way but they look unfamiliar.

These icons should not change. With a Creator based on Qt 5.5.1, this works correctly.