Upgrade to Custom Elements V1 spec

[wkeese]

See ibm-js/delite#492 for background.

The API changes related to this change are:

Buttons

Convert Button, ToggleButton, and DropDownButton to autonomous custom elements.
(i.e. <d-button> instead of <button is=“d-button”>).

HTML referencing buttons (including templates) must make syntax changes:

• instead of
• onclick —> on-click


Additionally widget subclasses must be updated:

• add role=button to template
• CSS: :disabled —> [aria-disabled=true]

BoilerplateTextBox

Change BoilerplateTextBox#Field to be an autonomous element containing an <input>, rather than a customized version of an HTMLInputElement. User CSS may need corresponding changes.