Resources that are inherited from a superclass cannot be set
by functions like create-widget; they must be set after the
widget has been created by a call to set-values!.
This restriction is only there for the first widget of each class.

The reason for this is that the complete resource list is
not available (through XtGetResourceList) before the class
has been initialized.

Constraint resources also can only be set by means of set-values!.

Subresources, on the other hand, can only be set by functions like
create-widget (this is a restriction imposed by Xt).  Subresources
cannot be read with get-values.  In addition, converters do not
work for subresources (since XtConvert needs a widget instance).


Callbacks *must* return; e.g. a (reset) from within a callback is
not allowed.  This is a bug in Xt.


Missing: context-add-input, context-remove-input, accelerators
resource converters, keycode translators, case converters,
shared GCs, selections