set comment on plasma applet desktop file

Summary:
plasma applets automatically get appstream metadata generated from their
desktop file. the Comment entry is used for the summary which is a strictly
required field in the metadata.
set comment so that valid appstream metadata can get generated from the
applet metadata

Reviewers: kossebau, #marble, nienhueser

Reviewed By: #marble, nienhueser

Differential Revision: https://phabricator.kde.org/D7363