Finds the child with the input focus.
A pane or layout.
A pane or
pane-descendant-child-with-focus attempts to find the pane inside
that currently has the input focus, and returns this pane if successful.
pane-descendant-child-with-focus may return
nil if it does not find a pane with the focus.
CAPI User Guide and Reference Manual (Macintosh version) - 25 Feb 2015