Fix getScriptDirectory utility function

Use the calling script as a reference, not the file where
getScriptDirectory resides.
Linux install script for ogg library
Linux install script for Vorbis library
Add OgreOggSound sources to contrib

Due to some necessary changes to OgreOggSound's CMakeLists.txt,
it's easier to add its sources to contrib than to install it
into the build environment.
Custom CMakeLists.txt for OgreOggSound
Ignore OgreOggSound generated documentation in git
Add Find modules for sound libraries
Add OgreOggSound to compiled contribs
Move plugins into bin

OgreOggSound needs to be loaded as a plugin. But it also needs
to be linked to the executable, so the DLL must reside in the
usual Windows DLL search path. Which is the executables
directory. Therefore, all plugins need to move where the
exeutable is so everyone's happy.
Add SoundSourceSystem
Fix FindOpenAL

The previous version was a mess. The new version is pretty much
copied & pasted from FindVorbis.
Disable OpenAL EFX

OgreOggSound requires the efx-util.h header when EFX is enabled.
I can't find this header in any recent version of the OpenAL
library, so let's disable it for now.
Copy sound files over
Setup sound manager in engine
Make SoundSourceSystem::init take a GameState instead of an Engine
Add SoundSourceSystem::activate and deactivate
Add Lua bindings for SoundSourceSystem
Add test code for sound
Adapt sound branch to binary assets changes
Sound sources can now play multiple sounds