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