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.

To undraft (similar to !1250)

• fix TODOs (input region, move background creation out of shell)
• Improve Settings visiblity over unfolded overview
• handle high contrast
• osk shines through on the app screen when unfolded manually
• home bar is not rendered at full with 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)