HV-920 Adding a regular expression based URL constraint validator which ...

...needs to be configured via XML or via a ConstraintDefinitionContributor
> Ah, just seeing this package. So you consider that validator a public API? 
I am torn here. Initially I had it with the other validator implementations. But that means that if a user wants to use it as suggested in the docs, she has to reference an _internal_ class. That's obviously against our package design. For that reason I made it public API. On the other hand, if we would keep it _internal_, we could change/remove it more easily. One could add a note saying that this is "experimental" and open for change. I decided for the former approach.

> I guess it can be looked at that way, but I feel a bit uncomfortable about it.

Why?

> At least we'd have to move AbstractURLValidator here as well, as a public type must not inherit an internal one.

Why not? Conceptually I see no problem here, but on second thought there might be a problem with OSGi.