fix RTL appearance for ComboBox

Summary:
--PlasmaComponents3.ComboBox now appears correctly
its list items in RTL environments by aligning them
to the right

BUG: 387558

Test Plan:
check the combobox list both in RTL and LTR environments
in order to confirm that the list items are aligned correctly

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: mart, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D9275