Panes help arrange items, but the real work of the user interface is
done by items. Most items are used to get input from users though
a number show output and some do both.
items cannot contain other items or panes, they are the leaves of
the ui tree.
The full path to each item class is thing.ui.item.the_item_class,
though each item class, like each pane class, has a shortcut
of just ui.the_item_class.
See the doc on ui for fields common to all ui objects.
the_value
Most items have a field called "the_value".
For checkboxes and radiobuttons the_value is true or false depending on whether it is
selected or not. For textfield and textarea it is the text typed in, etc.
A few items like button and menuitem don't have the_value fields.
Usually the_value is read by looking and set by the user clicking or typing.
All the_value fields can also be read using Water's method "get" or
the dot syntax for getting a field value.
the_value can be set using Water's method "set".
the_value behaves just like a regular field except that when it is set,
the underlying native_object used to display the item is updated so that
you can see the new value.
click
Most items have a click field. Its value is a user defined method that
is called whenever the user clicks on the item. The method takes no arguments
and need not even have a name. It is called with the item that was clicked on
as its subject so that all the fields of the item including its name and the_value
are available to the method for introspection.