Summary:
Adds XDGV6 support for the kwin side.

Popup placement support is limited to the stuff v5 had,
a simple offset, rather than the awesome new positioner.

But Qt doesn't make use of it yet either.
Also ideally we should do all the positioning before sending the first
configure, but again Qt doesn't actually do anything with that anyway.

Also integrate pinging clients

Test Plan: gtk3-demo  works nicely.

Reviewers: #plasma, graesslin, mart

Reviewed By: #plasma, graesslin

Subscribers: mart, graesslin, kwin, plasma-devel, #kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6591