Jump back in the background on pause.

This moves the logic from onPlayBackResumed to onPlayBackPaused.  A timer is
started there to jump back at the desidered (calculated) percentage (there's no
SeekTime built-in function) and it is cancelled when manually resumed or
playback is stopped (the call to SeekPercentage is ignored if nothing is
playing, but this is needed for e.g. playlists).

On a device like a Raspberry Pi the previous logic randomly caused the following
behavior:
  * user resumes playback;
  * video plays for a second or two;
  * it finally jumps back.

Although I'm not sure I believe this behaves as intended, as Python threads have
a lower priority than player's ones.

Additionally, in case of a high bit rate video it may need a couple seconds to
be cached.  By rewinding while still paused it is cached "in the background", so
the user hits the resume button and it just plays.

--
From a quick look at Kodi/XBMC code, these built-ins work up to Eden (didn't check earlier versions).
Note: no need to handle onPlayBackEnded as that would mean that onPlayBackResumed was called before and the timer gets cancelled there.
I'll revert the version bump if you want to manage versioning yourself.