Drop dependency on Qt Quick Controls 1

Drop dependency on Qt Quick Controls 1

Qt Quick Controls 1 where deprecated with Qt 5.11 [1]. Nextcloud still
depends on Quick Controls 1 by importing QtQuick.Dialogs. Removing
those imports will cause the QMl script to use the according API from
Quick Controls 2, which are, fortunately largely API-comptible.

This helps distribution to deprecate and remove Quick Controls 1 [2].

1: https://doc.qt.io/qt-6/qtquickcontrols-changes-qt6.html#migrating-from-qt-quick-controls-1
2: https://bugs.gentoo.org/889772

Signed-off-by: Florian Schmaus <flo@geekplace.eu>