Port KCheckComboBox from KComboBox to QComboBox

Summary: We don't need any of the KComboBox/KLineEdit features here.

Test Plan: Used the gui test program

Reviewers: kfunk, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D28912