Fallback for cached icons

Summary:
Apparently some appstream metadata might have icon information in form of
<icon height="128" width="128" type="cached">org.gnome.gedit</icon>
instead of
<icon height="128" width="128" type="cached">128x128/org.gnome.gedit</icon>
which leads into missing icon as we are looking into wrong location.

Reviewers: apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7178