add backdropClick @Output in dialog

[maxime1992]

Bug, feature request, or proposal:

Sidenav has an @Ouput backdropClick :

This is very handy when using ngrx for example so we can dispatch an action and keep a trace in UI reducer of the sidenav.

It would be very convenient to have such an output with the dialog component.

[fxck]

#3251 / #2855

[kara]

Yeah, looks like dupe of #3251. We'll keep tracking it there (feel free to comment to add more info), but thanks for reporting.

[fxck]

@maxime1992 #3460

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