Merge the ODRS plugin into the core code

This is one step towards being able to rework the threading model of gnome-software. By reducing the interactions between plugins, it will become possible to change how the plugin vfunc calls are threaded. At the moment, the ODRS plugin adds reviews to the app list results from earlier plugins. By moving the ODRS code into the core of gnome-software, that interaction is removed.

This means the ODRS functionality will not be disableable at build time any more, although it can still be disabled at runtime by setting the review-server GSettings key to the empty string.

It also means that it will be harder to support non-ODRS sources of ratings and review data, but that doesn’t seem to be an issue because thankfully everyone uses ODRS (or nothing).

Signed-off-by: Philip Withnall pwithnall@endlessos.org