shell-app-system: Do not compare startup-wm classes with full desktop IDs

When fetching the desktop ids into a map of startup-wm classes we meant
to give the ones that match the desktop ID more priority, however
this did not happen because we were always comparing a desktop
file id, including the `.desktop` suffix, with a wm-class that generally
does not include that.

This is the case of gnome-system-monitor, that provides two desktop
files, one of which is OnlyShowIn=KDE but both have the same
StartupWMClass and thus the first parsed is preferred, even though its
desktop-id is gnome-system-monitor-kde.

As per this, remove the .desktop suffix when comparing it with the
startup-wm-class, keeping the old check just in case.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2721>