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.
Some menus are broken, but they are broken with wl-shell integration too.