Clip by bounding delegate size instead of Item.clip.

Summary:
* "Never enable clipping on a delegate."
  (http://doc.qt.io/qt-5/qtquick-performance.html)
  It mucks with the scene graph's ability to optimize draw order.

* Also means window rects don't get cut off, their frames are
  nicely bounded. Technically this is a small information loss -
  you can't tell from the pager anymore when a window is going
  out of screen bounds - but it's visually rather pleasing.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4302