fix RTL appearance for ComboBox

Summary:

• when the combobox isnt inside the main Item

but rather a sub-Item it doesnt inherit the
LayoutMirroring properties. This is a workaround
to handle that behavior.
QTBUG: https://bugreports.qt.io/browse/QTBUG-66446

Test Plan:
checkout that LayoutMirroring works properly in
such case

Reviewers: Plasma, mart, safaalfulaij

Reviewed By: safaalfulaij

Subscribers: safaalfulaij, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D10592