Simplifying HID creation.

Part of #296.

src/xenia/hid/input_system.h

@@ -13,9 +13,14 @@
 #include <memory>
 #include <vector>
 
-#include "xenia/emulator.h"
+#include "xenia/cpu/processor.h"
+#include "xenia/memory.h"
 #include "xenia/xbox.h"
 
+namespace xe {
+class Emulator;
+}  // namespace xe
+
 namespace xe {
 namespace hid {
 
@@ -26,6 +31,8 @@ class InputSystem {
   InputSystem(Emulator* emulator);
   ~InputSystem();
 
+  static std::unique_ptr<InputSystem> Create(Emulator* emulator);
+
   Emulator* emulator() const { return emulator_; }
   Memory* memory() const { return memory_; }
   cpu::Processor* processor() const { return processor_; }