mdBottomSheet: add md-bottomsheet-focus directive

[dmackerman]

It would be wonderful if $mdBottomSheet had an open to set focus upon opening, just like $mdDialog. I'm aware that this breaks screen readers, but we would like to set focus manually versus not having the choice.

FWIW: the focusOnOpen docs from mdDialog:

focusOnOpen - {boolean=}: An option to override focus behavior on open. Only disable if focusing some other way, as focus management is required for dialogs to be accessible. Defaults to true.

[ThomasBurleson]

Recently we added a md-sidenav-focus directive to specify which element in the SideNav area should receive focus when the SideNav element is opened.

I think something similar for dialogs and bottomsheet would be very useful.

[marcysutton]

That would be a good addition. It already does set focus on the first item in the bottom sheet, FWIW.

[ThomasBurleson]

@marcysutton - true. With md-dialog-focus and md-bottomsheet-focus these would allow specific customized element focuses.

[juanpujol]

+1

[dmackerman]

Going to submit a pretty simple PR for this.