ControllerInterface: Don't pass m_devices to the backends

Previously, the devices vector would be passed to all backends. They
would then manually push_back to it to add new devices. This was fine
but caused issues when trying to add synchronisation.

Instead, backends now call AddDevice() to fill m_devices so that it is
not accessible from the outside.

Source/Core/InputCommon/ControllerInterface/DInput/DInputKeyboardMouse.h

@@ -13,7 +13,7 @@ namespace ciface
 {
 namespace DInput
 {
-void InitKeyboardMouse(IDirectInput8* const idi8, std::vector<Core::Device*>& devices, HWND _hwnd);
+void InitKeyboardMouse(IDirectInput8* const idi8, HWND _hwnd);
 
 class KeyboardMouse : public Core::Device
 {