Qt: Prevent savestate window from opening while in multiplayer

CHANGES

@@ -16,6 +16,7 @@ Misc:
  - GBA: Better memory handling with PNG savestates
  - GBA Audio: Allow GBAAVStream to have no video callback
  - ARM7: Force disable LTO on two files to work around a GCC bug
+ - Qt: Prevent savestate window from opening while in multiplayer
 
 0.3.0: (2015-08-16)
 Features:

src/platform/qt/Window.cpp

@@ -684,6 +684,10 @@ void Window::openStateWindow(LoadSave ls) {
 	if (m_stateWindow) {
 		return;
 	}
+	MultiplayerController* multiplayer = m_controller->multiplayerController();
+	if (multiplayer && multiplayer->attached() > 1) {
+		return;
+	}
 	bool wasPaused = m_controller->isPaused();
 	m_stateWindow = new LoadSaveState(m_controller);
 	connect(this, SIGNAL(shutdown()), m_stateWindow, SLOT(close()));