Draft: Allow to set lockscreen wallpaper

Images are always loaded using cover style to make sure the whole background is covered. We don't update the wall paper while the screen is locked. We only load images on the lockscreen, ignoring shields.

Background can be set via

  gsettings set org.gnome.desktop.screensaver picture-uri 'file:///file/to/background.jpg'

I'll revisit the way the actual wallpaper is loaded when fixing #910 (closed) so we can avoid for the XML and also more modes.

@snwh this slightly changes the settings margins, just in case you want to double check.

TODO

• land !1262 (check there for details)
• load image async (we can use another separate layer-surface as we do with settings)
• handle HC theme switches (e.g. to HC)