Switch the Plasma Desktop KCMs to using KNewStuffQuick

Summary:
This switches the KCMs which previously used a bit of a hack to show the old Widgets based Get Hot New Stuff dialog from a button in the Qt Quick based KCMs so that they instead use the new KNewStuff Qt Quick submodule. The modules affected by this are:

* Colors
* Cursor Theme
* Desktop Theme
* Icons
* KSplash

Reviewers: mart, ngraham

Reviewed By: mart, ngraham

Subscribers: bcooksley, davidedmundson, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26544