ControllerInterface: Add a way to register callbacks

This adds RegisterHotplugCallback() to register a callback which will
be invoked by the input backends' hotplug threads when there is a new
device, so that Core (GCKeyboard, GCPad, Wiimote, Hotkey) can reload
the configuration without adding a dependency to Core from InputCommon.

Source/Core/Core/HW/GCKeyboard.cpp

@@ -36,6 +36,7 @@ void Initialize(void* const hwnd)
   }
 
   g_controller_interface.Initialize(hwnd);
+  g_controller_interface.RegisterHotplugCallback(LoadConfig);
 
   // Load the saved controller config
   s_config.LoadConfig(true);