Add Display::Panel tests, tweaking Display::Panel .

Test main functionality of `Display::Panel`, including the building of
subpanels from structure definitions. Such tests are brittle, but do at
least force a complete run through the structure-building process,
checking co-ordinates with division of rows into differing numbers of
columns, also with dividers taken into account.

Adjust `Display::Panel` to use empty structure if none specified, also
exposing non-internal attributes using readers.
Expose non-internal attributes for Display::Screen,Display::Subpanel .

For those attributes not considered 'internal' (attributes which the
outside really has no business reading), expose them using readers.
Simplify tests, causing the readership to be tested.
Update README Screenshot and Growing Like Flowers.