Improve the behavior when pressing left/right/up/down on the
corresponding edge of the grid - wrap around to the previous or next
column/row.

In practice, this means that all layouts can be navigated with either
up/down alone or left/right alone, which is important for devices with
few hardware buttons, as they may only have one set of these keys.
This works well because fbwhiptail never actually creates an MxN grid
of items, it is always a single column (menu), a single row of two
items (yes/no), or a single item (msgbox).

Internally, fbwhiptail had two layout modes - dynamic-columns and
dynamic-rows.  Dynamic-columns fills rows vertically and creates as
many columns as needed for the items.  Dynamic-rows fills columns and
creates as many rows as needed.

Dynamic-rows was practically unused.  The only mode that used this
layout was "yes/no", and it has exactly two buttons, so it can use
either layout with no difference.  "Yes/no" no...