Input: Fix default key bindings
Perftimer: Make GPU query count configurable
Add patch for enabling pkg-config support in SDL_sound
Audio: Replace SFML solution by using OpenAL directly

This is a major change in the Audio module that comes with
many changes throughout the codebase and dependency list.
The main gist is that we're finally nuking the last pieces
of SFML from the project. sfml-audio brought with itself
unneeded and big dependencies (libsndfile, libvorbisenc)
while at the same time limiting the amount of audio formats
mkxp can support (eg. we now get mp3 for free, and wma/midi
can be implemented by extending SDL_sound directly).

The increased control gained by interfacing with OpenAL directly
will also allow for easy integration of a dedicated audio
stretcher (librubberband), as well as enable us to implement
looped ogg vorbis (via the 'LOOPSTART'/'LOOPLENGTH' tags),
as required by RGSS2, in the future.

The FileSystem class has had its SFML parts removed.
Aditionally, audio file extensions to be supplemented are
now automatically detected based on how SDL_sound was
built (ie. if no mp3 support was built, mkxp won't try
to look for *.mp3 files). The final used extension
can be optionally returned by 'openRead' calls so
SDL_sound and SDL2_image can immediately choose the
right decoder.

The OpenAL context is created and destroyed in main.cpp
along side the GL context.
Bitmap: Feed image extension into SDL2_image for faster loading
MRI-Binding: Silence "may be uninitialized" warnings
README: Reflect recent audio changes