Massive refactoring of all code + audio skeleton.

This should make it easier to find files and (in the future) split things
up into separate libraries.
It also changes around emulator initialization to make it a little more
difficult to do things out of order and a little more sensible as to when
real init work happens.
Also adding a skeleton audio system/driver and reworking CPU register
access to be more extensible.

src/xenia/gpu/graphics_driver.cc

@@ -24,7 +24,3 @@ GraphicsDriver::GraphicsDriver(xe_memory_ref memory) :
 GraphicsDriver::~GraphicsDriver() {
   xe_memory_release(memory_);
 }
-
-xe_memory_ref GraphicsDriver::memory() {
-  return xe_memory_retain(memory_);
-}