Fix button menus not opening on the second attempt

Assign a parent to the QQmlComponent returned by
QQuickControlSettings1::styleComponent() so that it doesn't
get garbage collected by the QML engine. This was not
an issue until 5.12, but it seems like a good idea regardless.

Change-Id: I53265b23afab62e2276fe6e10d976a93a4f12e6f
Fixes: QTBUG-71238
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>