Introduce Applications panel

This is an initial implementation of most of the
intended functionality for this panel. Flatpak
integration itself is implemented by spawning
"flatpak info -m $appid", which gives us the
metadata in key file format, and allows flatpak
to be a runtime dependency.

Even after removing the .desktop suffix, there can
still be a difference between the app ID generated
in this way and the flatpak ID, since flatpaks are
allowed to export files whose prefix is the flatpak
ID. Fix this by pulling the X-Flatpak key out of
the desktop file. This would cause trouble for
org.libreoffice.LibreOffice-math.

shell/cc-panel-list.c

@@ -386,6 +386,7 @@ static const gchar * const panel_order[] = {
   "universal-access",
   "online-accounts",
   "privacy",
+  "applications",
   "sharing",
   "sound",
   "power",