Vertical placement of digits made conditional

The fix to move the digits down to centre them caused the numbers to
become too tall when on a nest or in a box. Fix now only applied when
unconstrained by container.
Moved 'description' to JSON semantics

Not really a view attribute. Operational for robots, birds, and nests.
Numbers now support descriptions
Bird and nest copies have descriptions

If the originals did.
Boxes now have descriptions.
Element widgets support descriptions.
Titles include descriptions.

All widgets (earlier only robots) included descriptions in their titles
(tool tips).
Top-level backside has get_description

Though it does nothing but is called.