Redesign Window Switcher layout design

Complete rewrite of the dialog to select the window switcher
layout to fix the usability and scalability issues of the
existing implementation:
* added search functionality through QSortFilterProxyModel
* increased the dialog size to better render large layouts
* added scrollbar from decoration kcm
* "normal" list background and selection instead of window
  background
* only show one preview at a time

BUG: 299278
FIXED-IN: 4.9.0

current remaining issues:
* small icons looks really bad
* initial selected layout is broken, fixed after forced re-rendering
* grid layout does not use all available space
* highlight moves too slow