Install the gwenview part into kf5/parts, embed JSON metadata

Summary: lxr says nobody is loading "gvpart" by name.

Test Plan: kparts/bin/partviewer /tmp/myfile.jpg still works, after removing the old gvpart.so

Reviewers: Gwenview, kossebau, ngraham

Reviewed By: Gwenview, ngraham

Subscribers: ngraham

Tags: Gwenview

Maniphest Tasks: T12340

Differential Revision: https://phabricator.kde.org/D26349