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/xenia.h

@@ -12,9 +12,14 @@
 
 #include <xenia/common.h>
 #include <xenia/core.h>
+#include <xenia/emulator.h>
+#include <xenia/xbox.h>
+
+#include <xenia/apu/apu.h>
 #include <xenia/cpu/cpu.h>
 #include <xenia/dbg/debugger.h>
 #include <xenia/gpu/gpu.h>
 #include <xenia/kernel/kernel.h>
 
+
 #endif  // XENIA_H_