WMF: fixed MediaPlayer buffering logic.

To have a consistent behavior with other backends, the WMF plugin now
starts the session after loading a media in order to start buffering some
data and correctly notify when the media is buffered.
It was previously reporting a BufferedMedia status only (and wrongly)
after explicitly starting the media player.

Not all source readers (usually a source reader is specific to a file
format) implement the service needed to query buffering progress. In that
case just report the media to be buffered immediately after loading.

Change-Id: I6e6332ae08e96fc789556761e5169b88c36c5e37
Reviewed-by: Christian Stromme <christian.stromme@digia.com>