editable property-attributes

Run ceci's index.html and in the console type `$("#input").buttontext = "new text"`. Magic - the button innerHTML changes, *and* the <app-input> element now has an attribute called `buttontext` with that same string. Changing the attribute by hand will also change the button's innerHTML. `defaulttext` has a similar handling.

To get the definition for UI work: `$("input").getAttributeDefinition("buttontext")` or `$("input").getAttributeDefinition("defaulttext")`.