Sorting and disabling functionality for ExplanationServices

[stefborg]

This pull request modifies the general Explanation preferences. Users can choose between (a) always using the most recently used explanation service when requesting a new explanation (current behavior and default) and (b) using the explanation services according to a user-defined order. Additionally, individual explanations services can be disabled in the preferences. The reason is that a user may always want to compute justifications first (because it may be faster), and only request more expensive explanations when the justifications aren't enough.

This came out of a discussion on liveontologies/protege-proof-explanation#2, where this functionality would be useful for sorting and filtering many different proof services (e.g., supplied by https://github.com/liveontologies/elk-reasoner or https://github.com/de-tu-dresden-inf-lat/evee). It makes sense if this functionality is not only implemented at the level of proof services, but also in general for all explanation services. The proof service preferences can then also use the configuration options (a/b) described above.

For now, I tested this with the two explanation services https://github.com/protegeproject/explanation-workbench and https://github.com/liveontologies/protege-proof-explanation, but a similar implementation was tested for 10+ proof services.