Don't emit palette changes if the palette didn't change

Summary: Should keep us from repainting objects that depend on it.

Test Plan: Detected it with KObjectTracker, happens several times on every startup.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D27345