Mark featured AppStream snippets as append-merge components

The components in collection XML have to be marked with a merge mode explicitly if they are expected to be joined together with other component metadata. Otherwise they will be read as separate new, but invalid, generic components. This change ensures the GNOME Software metadata additions get registered no matter which AppStream implementation is used.

Without this change, as soon as GNOME Software is installed, every software center that isn't using appstream-glib will spew out a lot of warnings about invalid metadata. On Debian/Ubuntu this also is a bit noisy, as the AppStream reference implementation things that there is something broken in the data provided by the distribution. It's not critical, just a bit annoying ^^

Refer to https://www.freedesktop.org/software/appstream/docs/chap-CollectionData.html#spec-asxml-tags for a specification of the different merge mode and the purpose of merge components.

@hughsie I see no way this can cause issues, but still would feel safer if you could have a look and ACK it - pushing it directly feels wrong.