[Lxqt] Qt6 builds of lxqt components

I have created the subproject X11:LXQt:Qt6 which is building the Qt6 versions of lxqt, as they come available. These are *very* wip upstream, and things are likely to be broken. They are also not currently co-installable with the standard Qt5 versions of lxqt, so install at your own risk. https://build.opensuse.org/project/show/X11:LXQt:Qt6