adwaita-qt6/adwaita-qt

adds the qt6 version of the adwaita-qt theme. qt6 apps need this to use adwaita in qt.
i tried packaging this as part of adwaita-qt, but splitting -dev out to two separate packages is a bit of a mess, and it's easier to do it as a separate package.
could also have adwaita-qt-dev have both qt5 and qt6 of the dev dependencies as well, instead, if it's preferred to have them joined.