Add ability to prevent window closing

[BenLambertNcl]

In OpenFin, listening to close-requested disables the closing of the window. @jleft suggested this approach instead:

I'd propose that the behaviour by default would be that the window will close, regardless of whether the close listener is added or not (in OpenFin the close would be forced). However, add the option for the user to prevent the default behaviour (i.e. prevent the close using event.preventDefault()) within their event handler. Our API can then detect if default has been prevented (i.e. event.defaultPrevented) and use this to determine whether this window should be closed or not (in Openfin, force close if event.defaultPrevented === false, otherwise don't close). What do you think?