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.
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).
