gs-appstream: Add support for <supports> element

This is another element in the same family as <requires> and <recommends>.

It has a slightly different meaning from <recommends>, but for now the gnome-software code treats it as identical, as it’s new to the appstream spec and many apps which might want to use it are probably still using <recommends> instead.

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

Fixes: #1774 (closed)