ScreenType is being overloaded in incompatible ways:

- the editor sets "system_menu" to trick StepMania.cpp into not
   allowing the operator menu button, to prevent losing data;
 - the editor also sets "gameplay" in "playback" mode to influence
   ScreenSyncOverlay and ScreenDebugOverlay; this breaks the above,
   allowing scroll lock in play mode.

A similar problem happens when we push screens: scroll lock works
in the MiniMenus in the editor.

It's semantically unintuitive to call the editor a "system_menu", anyway.

Split out AllowOperatorMenuButton, and use it directly.  Check all
screens, and don't allow it if any screen on the stack disallows it.

stepmania/src/ScreenManager.cpp

@@ -283,6 +283,17 @@ Screen *ScreenManager::GetTopScreen()
 	return g_ScreenStack[g_ScreenStack.size()-1].m_pScreen;
 }
 
+bool ScreenManager::AllowOperatorMenuButton() const
+{
+	FOREACH( LoadedScreen, g_ScreenStack, s )
+	{
+		if( !s->m_pScreen->AllowOperatorMenuButton() )
+			return false;
+	}
+
+	return true;
+}
+
 bool ScreenManager::IsStackedScreen( const Screen *pScreen ) const
 {
 	/* True if the screen is in the screen stack, but not the first. */