ztix import does not update events

as noted by @dench0

to find out if performance node for event already exist it is also using value evid from xml. Previously it is searching via performance titla and date+time. BTW in code it is exist function for updating existed performance nodes, but it is commented with message # update is broken, so if it find already existed performance node for event with evid it will just skip it.

This should be fixed. At least the fields "info", "onsaleStopped" and "title" should be updated.