[WIP/RFC] Use Purpose for sharing

Summary:
Use Purpose for the sharing menu.

The question is what to do with the old KIPI sharing infrastructure. It covers some services that Purpose doesn't. Getting rid of them without a replacement would probably upset some users. Ideally all KIPI plugins should be ported to Purpose, but that's a huge task.

Test Plan: Open an image, Share it

Reviewers: Gwenview, ngraham

Reviewed By: Gwenview, ngraham

Subscribers: ngraham

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D21468