Fix Issue where Youtube plays when not in view

This is a more optimal solution to the previous changeset.  However,
same principle applies.  By removing the Loaded event from the WebView,
we make it eligible for garbage collection.  We can collect it when the
FlipView's selection
changes which will by design queue up a call to WireUpWebBrowser.  Note:
the FlipView holds the next and previous in memory so they aren't
collected.  Thus you can play two videos next to each other in a group
at the same time.  This is potentially desirable.