Reduce the amount of spurious property changes on ColorScope

Summary:
At the moment whenever something changed we were emitting colorGroupChanged
and then every color would recompute. This would end up being emitted
over 10 times at plasma startup so far.
This patch makes sure that the property will only be emitted if the color
group actually changes.

Test Plan:
Ran plasma, didn't notice issues.
I don't see all of the changes on the property anymore

Reviewers: Plasma, mart, davidedmundson

Reviewed By: Plasma, mart

Subscribers: davidedmundson, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D8917