Reviewed by Dave Hyatt and Adam Roben.

        https://bugs.webkit.org/show_bug.cgi?id=26278
        Patch that make WebCore have a RenderTheme per page

        Create a different RenderTheme per page, so that RenderTheme has
        access to page specific theming. This is needed for the Qt port, as Qt
        supports setting the theme (style) per widget.

        This change was suggested and discussed with Dave Hyatt.

        More detailed:

        1) Create a theme per page or one global one, depending on the needs
           of the platform.
        2) Add an accesser to the theme from RenderObject.
        3) Change all uses of the theming to access the theme through
           RenderObject, using the global default theme as fallback, when the
           document of RenderObject has no page.
           When we don't have access to a RenderObject, use the default theme.
        4) Modify all RenderTheme platform implementations to work with the
           above changes, still creating only one global theme.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@44758 268f45cc-cd09-0410-ab3c-d52691b4dbfc