Reduce dependencies on Motif and Xaw

Some parts of OpenPHIGS depend on Motif and Xaw. Both are supported, and one can switch between them at compile time.
Both are rather old, and support for them is vanishing, hence it is time to think of a third alternative. A possible replacement is gtk4.
It would be nice to start to add support for this, with the ability to switch between Motif and GTK4 at compile time, keeping the support for Xaw and Motif, with Motif as default for now.

No files, tests, or entry points are named; first map where Motif and Xaw are selected at compile time. Done means adding a GTK4 alternative while retaining Xaw and Motif support, with compile-time switching and Motif remaining the default.

Written by the indexing model from the issue text.