Built-in support for enter key

[fredericDelaporte]

#724 is a great addition, and thanks for that.
Now that the slider can have the focus, I think it should also trigger the form default button when an user hit enter, if the slider has focus and lies within a form, as would do most form fields.

Details about implicit form submission are here. The default button is the first submit button in document tree order in the form, even if disabled. The implicit submission implies triggering the click on the default button, if there is one and if the button is enabled, otherwise do nothing. The form should not be directly submitted. (It would cause it to miss the default button name/value pair in the submitted data.)

Maybe there will be some subtleties to handle. Like if the update event fires on blur, special care should be taken to ensure any pending update has been fired before triggering implicit submission.
And it may not be that easy to pick the default button: as far as I understand the spec, it would be first among input[type=submit], button:not([type=reset]):not([type=button]), in css notation. On button tag, a missing or invalid type defaults it to be a submit button.