Don't recreate backgrounds all the time

Based on @marty1885 's work in !531 (merged) i removed the recreating of backgrounds by listening to PhoshMonitor::configured and PhoshShell::notify::primary-monitor. This also includes the suggested cleanup of PhoshBackground as suggested by @sebastian.krzyszkowiak in !531 (merged). Since we wait for the final configure i don't think we need !541 (closed).

This way we make sure PhoshBackground only needs minimal knowledge about what's going on with PhoshMonitor and ideally we remove the phosh_shell_get_usable_area () at one point as well so this class an focus on rendering the background.

Closes: #325 (closed)