A rather big change to how tracks are loaded: there are now two parts to the process...

A rather big change to how tracks are loaded: there are now two parts to the process and metadata loading is done by the buffering thread (except for the first unbuffered track). The audio thread now calls audio_load_track, and once the metadata is loaded, the buffering thread sends an event which will make the audio thread call audio_finish_load_track. This one then takes care of the rest of the loading.
This method makes skipping noticeably faster for unbuffered tracks, and especially backwards.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@17109 a1c6a512-1295-4272-9138-f99709370657