New Preload System using a queue and a stack

This version of Chromesthesia pre-loads all songs that are uploaded one
after another, and when we swap to a new song, we add the index of that
song to a stack of the history.  When we go back, we pop off the stack
which song we are going to.

This method is much faster, and has much fewer bugs.  I need to refactor
the code still, but I think this branch will be merged into dev, and dev
will be renamed to "old" or something like that.