Add missing descriptions on the trashcan and appmenubar plasmoids

Summary:
Was missing, error appstreamtest and appeared empty on the Widget
explorer.

Reviewers: davidedmundson, #plasma, broulik

Reviewed By: davidedmundson, #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4205