Replace SDL2_mixer with Vorbisfile#

This pull request is here to help developers easily view the changes that this feature will make.

(This is an update of #256 for the FNA branding changes.)

Note that a new Git submodule is added in this patch. You will need to run git submodule init; git submodule update to download the new repository after checking out this branch, and eventually, when you update your local FNA branch.

This is for issue #187; you should read that issue if you want the details. Comments should also be directed to that issue unless you wish to make inlined comments in the actual code changes.