Add form-associated custom element

[carmacleod]

Mappings for form-associated custom element:

No implied/default role.

Roles that may be used: application, button, checkbox, combobox, group, listbox, progressbar, radio, searchbox, slider, spinbutton, textbox

Global and role-appropriate aria attributes.

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Notes:

• application was included because it could represent a new type of form control
• group was included because it's the default role for <fieldset> (although group can also represent non-form-associated groups as well)
• listbox was included because of <select> multiple or size > 1
• html output is a form control that maps to role=status. This wasn't included because it's not clear whether allowing role=status (without other live region roles) is necessary.