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.h

@@ -23,7 +23,7 @@ class GraphicsDriver {
 public:
   virtual ~GraphicsDriver();
 
-  xe_memory_ref memory();
+  xe_memory_ref memory() const { return memory_; }
   xenos::RegisterFile* register_file() { return &register_file_; };
   void set_address_translation(uint32_t value) {
     address_translation_ = value;