GameWindow.AllowAltF4 property

This is remake of my last commit - now its clear and contains all changes which Steve described to me. 

"In some cases, for some application with monogame, usage of alt+f4 for closing window is not advisable. What if developer wants to decrease the number of legal ways for closing app or want to use this combo with KeyboardState for his/her own purposes ? (example: shows in-game closing dialog by pressing alt+f4 instead immediate shutdown) So, why not add new possibility for developers ? The property called GameWindow.AllowAltF4. By default its true => nothing changed for existing apps. It works on WindowsDX and all OpenTK derived projects, but not on WinRT. It does not affect performance, nor standart window closing procedure(by click). If name(AllowAltF4) is not desirable I rename it to your version - suggest."