preview header for background should be transparent

https://github.com/ubports/system-settings/blob/2a041c6fbc6d07dd4b82bb10cd5d0b7c604666ad/plugins/background/Preview.qml#L120-L129

new uitk sets header background color hiding previous work to make a transparent for the ease of previewing the background.

Also move giant buttons to header for better image preview experience, and more consistent page navigation