Extension points

[aalmiray]

Fix for #27.

• Migrated all private state to protected for Field, SimpleControl and their subclasses.
• Package private state was migrated to protected.
• Package private constructors were migrated to protected.
• There were a handful of package private methods (persist, reset, validate) that were migrated to public as some of the subclasses had overridden the methods with public modifier.

These changes will enable consumers to create custom Field and SimpleControl subclasses without having to place their classes in the same package as the original FormsFX classes (a big no-no in modular codebases as split-package definition is prohibited!).

In my experience implementing a library with package private in mind makes it quite hard for consumers to fill in gaps with custom implementations, it would be better to keep our options open (protected) but not wildly open (public). Of course the usage of package private is a matter of taste and particular design goals that may not (or appear not to) be explicit.