appstream: Lookup app icons from .desktop files, if none found

The installed apps, coming from .appdata.xml files, usually do not have
set any icon. Try to lookup icons from the .desktop files in such cases,
thus, when the .appdata.xml files have properly set references between
then and their .desktop files, the expected icons are shown.

Closes https://gitlab.gnome.org/GNOME/gnome-software/-/issues/1514