Allow to define preference order of justifications plugins

[ykazakov]

In Protege preferences for justification it is possible to see all installed plugin:

It would be nice to be able to define the preference order in which these plugins should be used, e.g., the proof-based justifications is preferred to be used to the black-box justification.
The supported justification services displayed in the drop-down panel of the explanation window should be listed in this order.