Add methods to set automatic window closing

[kaikalii]

The implementors of AdvancedWindow that I have worked with automatically close the window when the X or ALT+F4 are pressed, but this is not always desirable. For example, upon the user attempting to close the window, a game might want to display a message first that says, "Are you sure you want to quit? All unsaved progress will be lost."

I have added methods to WindowSettings and AdvancedWindow that allow an automatic_close value to be set. If automatic_close is false, implementors of AdvancedWindow should not close the window, but should still send an Input::Close(..) event. The close attempt can be processed, and if the window really should be closed, then Window::set_should_close can be called. I have noted all of this in the method documentation. By default, automatic_close on WindowSettings is true so as not to add extra event-handling overhead for the majority of use cases.

[bvssvni]

Great work! Merge when ready.

[kaikalii]

Great work! Merge when ready.

Do you mean for me to merge it? because I don't have permissions.

[bvssvni]

Merging.