quickSettings: Add background apps menu

quickSettings: Add background apps menu

Sandboxed apps that run without a window are detected by the new
background monitoring service, introduced by xdg-desktop-portal.

We have an opportunity to improve the predictability of the desktop
and ensure that application state in transparently reported to users
by showing these apps, and allowing them to closed.

Add a new background apps menu to the quick settings, that is always
added at the bottom of the popover, and has a slightly custom, flat
style applied to it.

Show background-running apps in this menu, and allow closing them
by first attempting to execute the 'quit' action through D-Bus, and
if that fails, sending SIGKILL to the process.

See https://gitlab.gnome.org/Teams/Design/os-mockups/-/issues/191

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2624>