WMF: fixed various media player issues.
WMF: fixed various media player issues.

- Switch to BufferedMedia only once playback actually started, not when
requesting to start.
- Report the position to have changed when seeking in stopped state.

Change-Id: I930b3e6977cebe5935ed033d0a4d4e1eb899ad2c
Reviewed-by: Christian Stromme <0d8a7c35440bf88bf05c30d94f2a26326f5cfba7@digia.com>

mer-qt/qtmultimedia,mer-qt/qtmultimedia,mer-qt/qtmultimedia,mer-qt/qtmultimedia,mer-qt/qtmultimedia