[kcms/activities] Port to QQuickWidget from nested QQuickWindows
Nested QQuickWindows are still somewhat problematic especially when used in systemsettings which is already a very hybrid mix.
At some point this will all be fully ported to QtQuick anyway, there isn't anything especially complex going on in the UI that isn't QtQuick already. This is a small backportable patch to supress the bugs.