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);

Source/Core/Core/HW/GCPad.cpp

@@ -35,6 +35,7 @@ void Initialize(void* const hwnd)
   }
 
   g_controller_interface.Initialize(hwnd);
+  g_controller_interface.RegisterHotplugCallback(LoadConfig);
 
   // Load the saved controller config
   s_config.LoadConfig(true);

Source/Core/Core/HW/Wiimote.cpp

@@ -37,6 +37,7 @@ void Initialize(void* const hwnd, InitializeMode init_mode)
   }
 
   g_controller_interface.Initialize(hwnd);
+  g_controller_interface.RegisterHotplugCallback(LoadConfig);
 
   s_config.LoadConfig(false);
 

Source/Core/Core/HotkeyManager.cpp

@@ -186,6 +186,7 @@ void Initialize(void* const hwnd)
     s_config.CreateController<HotkeyManager>();
 
   g_controller_interface.Initialize(hwnd);
+  g_controller_interface.RegisterHotplugCallback(LoadConfig);
 
   // load the saved controller config
   s_config.LoadConfig(true);

Source/Core/InputCommon/ControllerInterface/ControllerInterface.cpp

@@ -172,6 +172,28 @@ void ControllerInterface::UpdateInput()
     d->UpdateInput();
 }
 
+//
+// RegisterHotplugCallback
+//
+// Register a callback to be called from the input backends' hotplug thread
+// when there is a new device
+//
+void ControllerInterface::RegisterHotplugCallback(std::function<void()> callback)
+{
+  m_hotplug_callbacks.emplace_back(std::move(callback));
+}
+
+//
+// InvokeHotplugCallbacks
+//
+// Invoke all callbacks that were registered
+//
+void ControllerInterface::InvokeHotplugCallbacks() const
+{
+  for (const auto& callback : m_hotplug_callbacks)
+    callback();
+}
+
 //
 // InputReference :: State
 //

Source/Core/InputCommon/ControllerInterface/ControllerInterface.h

@@ -127,7 +127,11 @@ public:
                        const ciface::Core::DeviceQualifier& default_device) const;
   void UpdateInput();
 
+  void RegisterHotplugCallback(std::function<void(void)> callback);
+  void InvokeHotplugCallbacks() const;
+
 private:
+  std::vector<std::function<void()>> m_hotplug_callbacks;
   bool m_is_init;
   void* m_hwnd;
 };