fix(datepicker): refactor datepicker toggle to support theming

[mmalerba]

@jelbourn The md-datepicker-toggle-icon could easily be replaced with an md-icon the only reason it's separate is so users don't need to load a font to use it. Would it be better to have datepicker module register an svg set with just a 'material:datepicker:toggle' icon for now? Other components could create similar sets if they need to include icons.

BREAKING CHANGE: [mdDatepickerToggle] attribute has been changed to md-datepicker-toggle element

fixes #5224
fixes #4934
fixes #4684

[ocarreterom]

Maybe can be:

elementRef.nativeElement instanceof HTMLButtonElement;

[yogeshgadge]

@mmalerba Wondering how does one use Fontawesome icon for md-datepicker-toggle-icon ?

[mmalerba]

@yogeshgadge just use a normal md-icon, md-datepicker-toggle-icon only exists so we can ship a default icon without having to require a whole CSS font library

[jelbourn]

FYI there's already 9 uses of this inside Google we'd have to update

[jelbourn]

What do you think of just having

<!-- Icon is baked into this -->
<md-datepicker-toggle for="minDatePicker"></md-datepicker-toggle>

If someone wants to have a custom icon, they just do a custom <button> with the close behavior. [mdDatepickerToggle]="minDatePicker" is longer than (click)="minDatePicker.toggle()", so it seems like there's not much sense in breaking up the toggle behavior and the baked-in icon.

For the baked-in version, all the things someone would typically want to set on the native <button> are taken care of by the component (content, aria-label, and type).

[mmalerba]

Changed to suggested API, PTAL

[jelbourn]

LGTM, just needs rebase

[andrewseguin]

Can you try rebasing? Github doesnt say anything but our script does

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