VLCMediaPlayer: update time and position implementation to libvlc v4

This removes the previously cached ObjC objects for the time and moves to a struct from libvlc that is updated by default at a maximum every 500 ms, often less. To save energy, the API user can configure an even more rare time interval.

However, time and position will always return the current values at subsecond precision based on interpolation against libvlc and the system time.

The currently known VLCKit API and its behavior including notifications and delegate calls remains unchanged.

Additionally, this MR adds a helper to VLCTime that renders a stringValue with sub seconds.

Depends on !207 (merged)