Let `update` on `FXFormBaseCell` be overridable

I really don't want to change much about `FXFormBaseCell`, but the new `[self class] == [FXFormBaseCell class]` kills a lot of the default settings. Made it overridable. To do that I had to expose `update` in the interface. Ideally that wouldn't be necessary, but objective-c doesn't have protected methods. I made a `ForSubclassEyesOnly` interface that would ideally be explicitly included by people (apple uses a similar method), but right now it's really only for guidance since everything is one file.