[windowswitchers] QML replacement for flipswitch and coverswitch

Rewrite of the flipswitch and coverswitch window switchers in QMl,
using the KWin Tabbox API and PathView to layout the thumbnails.

The use of common Plasma components, following the user's theme,
and lack of reflections gives the effects a more modern (breezy) look,
consistent with the user's desktop and the other tabswitchers.

BUG:443757
BUG:184874
BUG:404802
BUG:321324
CCBUG:330529
FIXED-IN: 5.24