background: Load images async

Instead of handling loading in the background (and potentially blocking drawing) load them async. For that we move the settings handling into the background manager (which also handles clearing the cache on settings changes).

As the background now calls back into the background-manager we finish background-manager's setup in an idle callback to ensure the background -manager is fully up when the background needs it.

This simplifies the background handling as the background-manager no longer needs to be concerned with the details of the layer surface.

This is needed for !1262 and !1250

Closes: #447 (closed), #910 (closed), #743 (closed)