GStreamer: correctly get metadata from live sources.

For some unknown reason, "iradio-mode" was set to false on the
source element, which was preventing new metadata to be received
when playing live streams.

Task-number: QTBUG-37640
Change-Id: Ib90297e81e26a99c3dfc753bdcd5cbd1ee2f6764
Reviewed-by: Christian Stromme <christian.stromme@digia.com>
WMF: improve changing playback rate on a media player.

When going back to normal rate after playing in fast forward (greater
than 2x), playback seemed frozen for some amount of time (up to 8
seconds).
When playing in fast forward, only key frames are shown, ignoring all
the others. When returning to a normal rate, the source reader will
usually be pointing to a key frame in the future compared to the
player clock position, meaning that all the frames in between won't be
shown until the player clock catches up with the latest key frame that
was read.

When leaving fast-forward, we now reset the position on the player to
force the source reader to point back to the frame at the current clock
position and avoid the seamingly frozen playback.

Also, emit playbackRateChanged() signal when changing the playback
rate.

Change-Id: I4f04f0f250083378e94fb4a47f9f917abeaaf24e
Reviewed-by: Christian Stromme <christian.stromme@digia.com>