EventInfo: const fixes.

EventInfo had some const methods that were returning non-const pointers to member data. This violates the spirit of const-correctness (and the thread-safety checker warns about it). Clean up by splitting the affected methods into const and non-const versions, returning const and non-const pointers, respectively.

This could in principle break some existing code. Known issues have been fixed/worked around, but it's possible something got missed.