WebKitGTK+ update

Updates webkitgtk-shell and enables HTML5 video support in WebKitGTK+.

GStreamer support has mostly been merged from https://github.com/Metrological/qtwebkit/commits/qt5.2.1-stable and is based on work originating from http://cgit.collabora.com/git/user/julien/WebKit.git/log/?h=rpi-wayland-ac2-eglimage.