Add 'beforeClose' Observable to MdDialogRef

[wulfsberg]

Bug, feature request, or proposal:

Feature request

What is the expected behavior?

Being able to start processing instantly when a user selects an option in an MdDialog.

What is the current behavior?

An 'afterClose' Observable is available which only fires after all animations are done.

What are the steps to reproduce?

The behavior is easily seen on the "Examples" page of https://material.angular.io/components/component/dialog

What is the use-case or motivation for changing an existing behavior?

In many cases, a dialog functions as a confirmation for some additional processing (such as deleting an item), which in turn asynchronously updates the interface (say, removing the item from a list once deleted serverside).
Having to wait for an animation to finish before launching the async server call introduces unnecessary latency in the UI.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Material 2.0.0-beta.6

Is there anything else we should know?

Delaying until after animation is a deliberate choice, as described here: #3890, but I find that reacting instantly is also a reasonable behavior, so I propose an Observable for each.

[willshowell]

Related to #3460

[jelbourn]

Yeah, having beforeClose is something we intended to do (which is why we called the existing one afterClose to begin with).

[willshowell]

Oh also #4647

[jelbourn]

Ah, tracking via #4647 (thanks @willshowell)

[wulfsberg]

The onBefore/closeAttempt/etc seems more concerned with how the user attempts to close the dialog, and the possibility of canceling that.
As long as it also makes the value of the choice reasonably easily available, it can pull double duty for my need, but it does seem to be semantically slightly different. (And apparently got somewhat mothballed over various standardization/accessibility issues).

[willshowell]

@wulfsberg You're right that #3460 is really just focused on how the dialog has been closed. I'd suggest mentioning in #4647 that a beforeClose observable should emit the closing value so that your use case doesn't accidentally get forgotten here

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