Decouple save states from UI.

It's the UI's job to decide how and what things are displayed, not Core's.
This also reduces reasons for linkage issues.

Windows/MainWindowMenu.cpp

@@ -400,7 +400,10 @@ namespace MainWindow {
 		g_Config.iInternalScreenRotation = rotation;
 	}
 
-	static void SaveStateActionFinished(bool result, void *userdata) {
+	static void SaveStateActionFinished(bool result, const std::string &message, void *userdata) {
+		if (!message.empty()) {
+			osm.Show(message, 2.0);
+		}
 		PostMessage(MainWindow::GetHWND(), WM_USER_SAVESTATE_FINISH, 0, 0);
 	}