Some APIs return arrays at the top. With this change, a customapi widget
can select the top level object/array.
This is particularly useful when combined with the `size` formatter, but
can also be used for APIs that return scalar values directly.
The `size` formatter returns the length of an array or string, or the
number of keys in an object.
Some APIs do not return a summary of their contents but only the full
list of contents. The `size` formatter can be used to still show such
count on a widget.
