Added 'Comment' option to metadata config-dialog

Summary:
BUG: 365620

DISCLAIMER:
I have no idea how this code really works. It was just a guess.

Test Plan:
compile baloo-widgets, run dolphin

Reviewers: Dolphin, ngraham

Reviewed By: Dolphin, ngraham

Subscribers: ngraham, vhanda

Differential Revision: https://phabricator.kde.org/D9700