[ComboBox] Set close policy to close on click outside instead of only outside parent

Ensures clicking the ComboBox itself closes the popup as it does with widget QComboBox.
While Qt documentation for Popup says that is the default, QQuickComboBox explicitly overwrites it to be CloseOnPressOutsideParent
instead for whatever reason.
For our usecase the CloseOnPressOutside behavior makes more sense.

BUG: 408950

Differential Revision: https://phabricator.kde.org/D22509