Summary:
This patch makes use of wlroot's DataControl interface to support
clipboard management.

Unlike wl_data_device clipboards are sent on every change to all
watchers.
If the data device has a selection set it updates immediately.

Because it was started a year ago it uses the existing style of
wrapping objects. The unit test uses the new approach.

Test Plan:
Updated kwin
used wlroot's wl-copy, wl-paste which are xclip replacements to
show that the clipboard updated correctly

Reviewers: #kwin

Differential Revision: https://phabricator.kde.org/D29330