Use the same icon mode calculation for comboboxes as for buttons

Summary: When hovering a focused combobox the icon was in Selected  state resulting in a wrong color.

Test Plan: Hover over a focused combobox that has an icon

Reviewers: broulik, #breeze, ndavis

Reviewed By: #breeze, ndavis

Subscribers: ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28908