Document tabris.ui as a Widget
Document tabris.ui as a Widget

The current documentation does not include the `find`method.
Since UI is implemented as a Widget, document it as such.

Change-Id: Ie87658ed3bda39021f85ed730ab5696128f4ab57

eclipsesource/tabris-js,eclipsesource/tabris-js,eclipsesource/tabris-js