[dev-qt/qtwebkit] Rework HTML5 A/V backend

 * Fixes bug #523518
 * Instead of simply depending on gstreamer, add a separate USE for
   gstreamer:0.10 to clearly specify which slot should be linked to.
 * Specifying multimedia while having gstreamer* selected does not
   enable the use of qtmultimedia. It will only be selected if we build
   with neither of both gstreamer versions