The PrintWindow still used an old style combobox.
This resulted in a Mac user not being able to specify
which clustered printer he wanted to send a slice to.
Using the new QtQuick.Controls and specifically setting
the currentIndex fixed it for Mac.
Now with unified style as agreed upon by Simon & Ian.
Rules:
- ID before all other props.
- All props before children.
- All props after ID in alphabetical order.
- Empty line between children.
- Semi-colons.
Note: I didn't touch the DiscoverUM3Action because that's it's whole own UI part.
