core: add player API

This API will replace the usage of input_thread_t from interface modules. The
player implementation continue to use input_thread_t in private. The goal is to
hide the input_thread_t API when every modules are switched to the player API.

TODO (all need to be fixed, for VLC 4.0):

 - Gapless: API is complete but not implemented (the player can play several
   medias in a row, but without gapless).
 - Position callbacks: still using the input_thread_t position that is really
   imprecise (notified every 250ms minimum, and sometime more, depending on
   pf_demux implementation).
 - Seek/discontinuity callbacks: when seeking, the player can still send
   position of the requested position, the actual position or the next position
   to come. This leads to UI inconsistency.
 - OSD messages should be display from input callbacks but it's not possible
   now since you can't know if the event come from the user or from the core.