[RTL] - align properly the selected text for RTL

Summary:
--align selected text correctly for
PlasmaComponents2.ComboBox taking into account
RTL.

BUG: 387415

Test Plan:
use ComboBoxes in RTL and LTR in order
to check a proper behavior

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: broulik, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D9126