No way to add in Rules data processors

Testing token replacements with the UI, I figured that Rules data processors are not added in by default, i.e. they are just ignored as long as no one activates them.

Given we have data filters the only use-case left seems to be the token processor. Thus, we should make it so that the processor a) is able to tell whether it should be enabled b) we need to save that information and c) allow them to show some help.

But finally, rules data processors should not be rules specific but a feature of the context mapping library.