playback service / metadata: update timing on major position changes

The OS automatically calculates the approximate playback position and time based on the position and the playback rate we provide. This calculation is off on major position changes. However, when updating the dictionary on every position intervall, this would be very costly. This approach tries to update it for 10 times and then gives up. This is enough for local media and most networks.

This fixes #1224 (closed).