Made left button down event available to UI event handler

Main menu is no longer triggered in game on left mouse button press when handler is processing UI events
https://forum.zdoom.org/viewtopic.php?t=59673

src/menu/menu.cpp

@@ -714,7 +714,7 @@ bool M_Responder (event_t *ev)
 			return false;
 		}
 		else if (ev->type == EV_GUI_Event && ev->subtype == EV_GUI_LButtonDown && 
-				 ConsoleState != c_down && m_use_mouse)
+				 ConsoleState != c_down && gamestate != GS_LEVEL && m_use_mouse)
 		{
 			M_StartControlPanel(true);
 			M_SetMenu(NAME_Mainmenu, -1);