pipewiredelegate: wallpaper cues for outputs

this adds a visual hint to the output delegates based on the visuals of the current desktop containment (IOW: the wallpaper, for the most part)

it uses a new grab function of plasmashell to load a screen grab of the desktop containment into a file and then use that as source for our delegate's background if applicable and available

part of the visual refresh of the screen share dialog.