[AppStream] AppStream 0.12.6 released!

[Matthias Klumpp]

A very small AppStream release which brings a few (speed) improvements
to the search functionality as well as the new support for release
artifacts.

Tarballs available here:
https://www.freedesktop.org/software/appstream/releases/

Version 0.12.6
~~~~~~~~~~~~~~
Released: 2019-03-02

Features:
 * Implement artifacts (Matthias Klumpp)
 * Search improvements (Matthias Klumpp)

Specification:
 * doc: help distinguish the 2 'languages' tags (Beuc)
 * spec: Document the new artifacts tag (Matthias Klumpp)

Translation:
 * Finnish
   - Ammuu5
 * Hungarian
   - Balázs Úr
 * Turkish
   - Kemal Oktay Aktoğan
 * Spanish
   - advocatux
 * Danish
   - scootergrisen