Don't set a custom palette if we have the same palette as the currently active one

Summary: Caused some icons not be colored correctly

Test Plan: Close KWrite with unsaved changes, all icons should be colored correctly

Reviewers: broulik, ngraham

Reviewed By: broulik, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28830