[media] Add sub-second resolution for the duration

[media] Add sub-second resolution for the duration

The duration in ClutterMedia is currently expressed in integer multiples
of a second. This does not offer enough granularity for media playback
that has fractionary durations -- 2.3 seconds; 1 minute, 23.4 seconds;
1 hour, 23 minutes, 4.5 seconds; etc.

The duration value should be expressed in seconds with a sub-second
granularity; for this reason, a double should be used insted of an
unsigned integer.

The semantics haven't changed: the :duration property still exposes
seconds.