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.
This is the first step in improving the widget definition process. By
forcing the default behavior to be the widget behavior (thus
discouraging inheritance that doesn't work in C anyway), the default of
jwidget_poly_event() is exposed to the contract. This will allow further
features in using events, specifically for more complex focus cases.