Implement detection of played files

[Petr Nejedly]

For audio / video:

• detect start
• detect end
• potentially detect % watched
• all per client

Images - doesn't make sense.

Console can (optionally) show what is being played on which device. This should also support MediaBrowser (perhaps per clientId).

We can also add additional events, like 'watched', to enable sending events to 3rd party services, like for #594.

This needs to work with:

• multiple byte-range requests for the same file
• a single request for the whole file
• multiple time-seek requests for the same file
• assume a client can only play 1 file at the same time (ie a new play event for a new file marks the previous playing file as stopped)
• for progress, % downloaded != % watched (a file can be requested to be fully downloaded in a single request and the download is finished much sooner than watching it).
• should replace the current 'already watched' functionality and perhaps work with Samsung Bookmark method (for progress)

Should start with a minimal implementation (start + stop), to satisfy trakt.tv and some notifications to the Console API.