Drop (ancient) OFLib support

Closes #271.
I may be missing something here since I'm not deeply familiar with gdraw.

If GWidgetGetControl(d->gw,cids[i]) returns NULL, is running GGadgetSetVisible on that result safe somehow? It seems to dereference its first argument unconditionally. Or are we sure that GWidgetGetControl(d->gw,cids[i]) returns non-null unless there is something horribly wrong elsewhere?