Set combobox popups to be modal

Summary:
This closer matches what we see on QWidgets where losing focus closes
the combobox.

BUG: 403403

Test Plan:
Opened a combobox
Clicked outside
It closed

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18596