Only show LANG=C descriptions from AppData files

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=709436
trivial: Move the location of appstream_app_get_locale_value()

We need this for AppData too.
Show the best locale when using AppData files for the long description

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=709436