Draft: Use background in overview

This is not meant to be merged yet but as I keep dragging this around and the phoc side is now merged let's have it here as draft.

This is based on !1253 (merged) and !1250 as those parts of these MRs were cherry-picked from this branch.

TODOs to undraft

(similar to !1250)

• fix TODOs (input region, move background creation out of shell)
• Improve Settings visibility over unfolded overview
• handle high contrast
• osk shines through on the app screen when unfolded manually
• home bar is not rendered at full width when e.g the OSK is pulled out or the app splash shows
• input-region is broken after screen rotation (hence apps don't receive input). Unfolding homebar "fixes" it
• avoid slight shift in height between desktop background and overview wallpaper (only noticeable when docked)
• server side window decorations are visible under top-bar (see phoc!570)
• Fix top-panel transparency when unfolded: !1250 (comment 2157537)
• Improve CSS in overview

  • Add (maybe semi transparent) background of running

    

  • Make pressed button background semi transparent