Restore "Check if group < LastGroup, as KIconEffect doesn't handle UserGroup anyway"

Summary:
This restores commit 49f8718446322b1722822f99f6b9f3b975ea723e,
that was reverted by mistake in commit
20f7137145f6b6fb38d2c3dd01a8443b8315cf41.

Reviewers: broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22809