Provide higher-level "form-type" Widgets

[japhb]

These include all the basics needed for doing forms, wizards, etc: radio buttons, checkboxes, selection boxes, text areas, etc.

[japhb]

Note for the future: It's likely that we'll want to do proper event bubbling at a lower level than these specialized widgets, because many non-form apps will want to watch events sanely as well. See issue #19 for more on input handling.