[Style KCM] Port to new design

This ports the widget style KCM to use a grid view KCM with live preview thumbnails and inline configure button.

* Icon and tool bar settings have been moved into a popup, similar to Icons KCM, to leave more room for the rather large thumbnails
* Overall cleanup of unused/defunct features and modernized codebase
* Support for non-Linux is dropped, i.e. it assumes "Breeze" as the default, no "native" (e.g. Mac or Windows style) as we want
  our apps to follow whatever other platform they run in, rather than offering this KCM there

Differential Revision: https://phabricator.kde.org/D24361