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

src/declarativeimports/plasmacomponents3/ComboBox.qml

@@ -110,6 +110,8 @@ T.ComboBox {
             currentIndex: control.highlightedIndex
             highlightRangeMode: ListView.ApplyRange
             highlightMoveDuration: 0
+            LayoutMirroring.enabled: true
+            LayoutMirroring.childrenInherit: true
             T.ScrollBar.vertical: Controls.ScrollBar { }
         }
         background: Rectangle {