Qt: Disable "Start" unless we are paused (it otherwise has no meaning and causes a crash).

src/citra_qt/main.cpp

@@ -249,7 +249,7 @@ void GMainWindow::ShutdownGame() {
     System::Shutdown();
 
     // Update the GUI
-    ui.action_Start->setEnabled(true);
+    ui.action_Start->setEnabled(false);
     ui.action_Pause->setEnabled(false);
     ui.action_Stop->setEnabled(false);
     render_window->hide();

src/citra_qt/main.ui

@@ -90,6 +90,9 @@
    </property>
   </action>
   <action name="action_Start">
+   <property name="enabled">
+     <bool>false</bool>
+   </property>
    <property name="text">
     <string>&amp;Start</string>
    </property>