Prepapre everything for enabling the PageCache for the Qt Platform:
    * Claim that we can cache pages
    * Create the FrameView FrameLoaderClientQt::transitionToCommittedForNewPage using
      the initial size of the viewport and stop creating it in the QWebFramePrivate::init

Differences to the Windows port:
    * attachToWindow/detachFromWindow is not called and is not (yet) part
      of Widget/ScrollView of the Qt platform. We might need that for plugin
      support in the future.
    * We store the margin's and scrolling flag inside QWebFrame and use it when
      creating the FrameView.

What is missing:
    * API to call pageCache()->setCapacity(XYZ);


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@31568 268f45cc-cd09-0410-ab3c-d52691b4dbfc