Xcu V1.4 has two "new" widgets -- versions of XcuSimple (XcuSimpleComp and
XcuSimpleCompConst) that allow composite and constrained composite widgets
to have picture frames or shadows.

XcuLabel now works for XtNbitmap resources.

XcuTbl now permits its children or layout to be changed after initialization.
(See the public functiions.)

Still big on the TODO list:

    Add additional constraint resources to XcuTbl so that children can
    be customized with respect to behavior on resizes.

    Add "at least N of many" and "at most N of many" semantics to XcuBmgr.

    Permit popup widgets in the widget layout description language.
    Incorporate XtAddInput as a public function.
