Filip Roséen authored Mar 21, 2017 and Hugo Beauzée-Luyssen committed Mar 29, 2017

This refactoring should not only allow for easier maintenance as the
code size has shrunk, it also implements a few advantages over the
previous implementation:

 - playlist_preparser_Cancel is now optionally blocking if the
   referred to item is currently being preparsed (required in cases
   where another action would race with the preparser, such as
   playback (as preparsing and playing an entity at the same time can
   lead to duplicate items in the playlist).

 - the congestion in terms of interacting with the preparser, and the
   preparsing itself, is lower. Meaning that we will finish a queue of
   items to preparse faster than with the old implementation.

Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>