Log Message:
appstream-glib: update to 0.7.18.

Version 0.7.18
~~~~~~~~~~~~~~
Released: 2020-09-07

New Features:
 - Add content rating system APIs from gnome-software (Philip Withnall)
 - Add "validate-version" command (Florian Müllner)

Bugfixes:
 - Allow timestamp in the future in validate-relax (Bartłomiej Piotrowski)
 - Don't ignore localized strings that are the same as original (Isaque Galdino)
 - Fix crash with invalid children of <ul/> (Fabian Vogt)
 - Lower the OARS/CSM mapping of sex-homosexuality/intense (Philip Withnall)
 - Properly initialize mutexes (Laurent Bigonville)
 - Test launchable tags in validation (Ian McInerney)