Support more meaningful options for dialog autoFocus

[jelbourn]

Currently, the autoFocus option of MatDialogConfig is a boolean, with the values meaning:

• false - default value, focused the role="dialog" element
• true - focuses the first tabbable element inside the dialog

This API is kind of confusing, since the dialog automatically focuses something regardless of what you do, so autoFocus: false doesn't really make sense.

Instead, we should change autoFocus to accept these values:

• dialog - focus the role="dialog" element
• first-tabbable - focus the first tabbable element
• first-header - focus the first header (h1 - h6) element in the dialog. It's still an open question whether this should force tabIndex = -1.
• A CSS selector- arbitrary CSS selector, the dialog would focus the first matching element (vis querySelector). Again an open question if this should force tabIndex = -1.

[zelliott]

I might call it first-heading instead of first-header (https://html.spec.whatwg.org/multipage/sections.html#the-h1,-h2,-h3,-h4,-h5,-and-h6-elements) (wouldn't want it to be confused with <header>).

I'll throw a vote in for forcing the element to be focusable. I feel like forgetting to add tabindex will otherwise be a super common gotcha and result in bug reports that these values aren't working as expected.

The logic could be something like:

1. If tabindex is specified, do nothing.
2. Else, set tabindex to -1, and add self-removing event listeners to remove the tabindex once the element is blurred or mousedown-ed. You don't want to keep the element focusable because it will mess with a user setting the sequential focus starting point via "click to move focus" (https://googlechrome.github.io/samples/focus-navigation-start-point/).

The logic to "force-focus" an element and revert it back to normal afterward could be pulled out into a common helper function.

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