shell/app: Honor XDG SingleMainWindow key

This is the upstreamed version of X-GNOME-SingleWindow which is now a
part of the XDG desktop file spec as of version 1.5; see
https://gitlab.freedesktop.org/xdg/xdg-specs/-/merge_requests/53

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2084>
(cherry picked from commit a13b2358)