Improve missing imports resolution

[onor13]

1. Possibility to resolve missing imports from XML Catalog and not only an ontology file.
2. Possibility to change the active catalog.
3. Fix: the classes hierarchy was not reloaded when adding “Direct Imports” in “Imported Ontologies” window

[buergerj]

I second that issue and like to add: failed imports should be at least generate some kind of notice or warning at the GUI level.
This is currently not the case, but according to console output, Protégé is aware of failed imports ("Failed to load imported ontology at ...").
This pull request #734 would at least help by adding a warning symbol which calls attention when warnings in the Protégé log appear.