Added toggleScreen function, using hot key to loop through all screens.

Comparing to switchToScreen, it is more handy since the user only need
to hit one hotkey.

src/lib/server/Config.cpp

@@ -1182,6 +1182,10 @@ Config::parseAction(ConfigReadContext& s,
 		action = new InputFilter::SwitchToScreenAction(m_events, screen);
 	}
 
+  else if (name == "toggleScreen") {
+    action = new InputFilter::ToggleScreenAction(m_events);
+  }
+
 	else if (name == "switchInDirection") {
 		if (args.size() != 1) {
 			throw XConfigRead(s, "syntax for action: switchInDirection(<left|right|up|down>)");