Export desktop file & icon in development builds

Previously, no desktop file, app icon or D-Bus service would be exported, because org.gnome.Software is not within the org.gnome.SoftwareDevel namespace of the Flatpak.

With this change, the development build shows up in the app grid & search and on the dash, with the "devel" icon already present in the source tree.

I have not tested a non-Flatpak build of this branch.