Let `update` and `setUp` 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.

In addition I made `setUp` exposed so you can, for instance, override just the colors of the textfield in a subclass of `FXFormTextFieldCell`.