Categories of widgets
=====================

Focusable vs non-focusable

Non-focusable widgets are essentially a decoration. Examples are WLabel
and WFrame. Focusable are widgets which can be tabbed and which can
accept user input, and the rest of standard widgets are such.

Widgets can be tested for focusability using w.focusable property.

Editable vs non-editable
------------------------

Focusable widgets are subdivided into editable vs non-editable. Editable
widgets contain state which a user can change. Non-editable widgets
are intended to execute actions on user interaction. Examples of
non-editable widgets are WButton, WMenuBar, WMenuBox.

Choice widgets
--------------

A subset of editable widgets are choice widgets. They deal with selecting
one of the items among a number of choices. WRadioButton, WListBox and
WDropDown are choice widgets. For consistency, it makes sense to treat
WCheckbox as a choice widget too, with choices being False and True.

Widget hierarchy::

  BaseWidget
    Non-focusable
      WLabel
      WFrame
    Focusable
      Non-editable
        WButton
        WMenuBar
        WMenuBox
      Editable
        Choice widget
          WCheckbox
          WRadioButton
          WListBox
          WDropDown
          WCompletionList
        Non-choice widget
          WTextEntry
          WMultiLine
          WAutoComplete