Allow custom backgrounds/colors, not only checkerboard on the Qt Quick Designer

Several apps can use transparent subitems and white shapes or text on them.

Try yourself

1. Create hello world
2. Create transparent box/item, make white text part of it
3. Make a custom item (Refactor->Move to separate file)
4. Try editing white text on the checkerboard bg, it's difficult, you need a custom bg in order to really "see" it.

Current workaround use a Rectangle, and change the color. But it needs to be reset again every time. Time consuming. I mark it as bug because it interferes with real work.