Implemented/added methods in GUI and SimulationController

1- Slight changes in the form designer
2- Changes in GUI.cs:
- called constructor for _controller and set its Gui reference;
- added method string filePath(int n);
- added method string saveMessage(string title, string message);
- implemented buttons and menu items methods, and subscribed menu call
"close" to Window.Closing event;
3- Changes in SimulationController.cs:
- Added property for _gui;
- close, save, saveas, load, startSimulation, pauseSimulation and
stopSimulation, removeCrossing, selectCrossing, addCrossing,
setCrossingProperty, clearGrid,  methods changed from bool to string;
- implemented method close.