This requires jfkeys to _get_ the events, which needs to be done
manually by its owner to some degree (in gintctl, gscreen handles it).
Currently there are just 6 values 0..5 for F1..F6, but future expansion
for rotating menus with more than 6 entries and nested menus might add
to this set.
We still have a few platform-specific occurrences that I'll want to get
rid of in the future. This should ultimately leave me with only one
version of the library (but this also precludes the use of macros like
DWIDTH/DHEIGHT which will require further changes).
