Expose property to retrieve validatable/submittable elements

[valdrinkoshi]

<iron-form> contains all the complex logic of searching for the validatable and submittable elements. It would be really helpful for it to expose a public method/property to retrieve these.
@notwaldorf WDYT about making _getValidatableElements & _getSubmittableElements public?

[web-padawan]

BTW, consider also creating separate helper similar to iron-focusables-helper. This would allow to re-use some logic of iron-form without pulling in the element itself.

[notwaldorf]

I don't have any huge objections to making them public, other that they will become part of the public API, and thus harder to maintain (no breaking changes, etc). Is there a benefit to having them public vs protected, like they are now?

[vdegenne]

@valdrinkoshi What about this type of helper that simulates native form.elements ? (#273)