Add plugin type for sharing URLs

Summary:
Add a plugin type for sharing URLs and adapt KDE Connect plugin to support it.
Other possible plugins would be email, twitter, various messengers etc.

Test Plan: Share page from Falkon D17286

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17285