Remove event: don't compare pointers, compare events

This used to work because we actually displayed the current_dive. Now with
displayed_dive the pointers are of course different! So we need to compare
the actual events instead.

See #616

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Rename event: correctly replace event with new one

I don't like that the event structure includes the variable length array.
That really makes it a pain to change the name of an event (on the flip
side, freeing events is easier I guess).

Anyway, to correctly rename an event we need to actually remove the event
from the correct dc and then add a new event with the new name. The
previous code was insane (it only worked if the new name was of smaller or
equal length, otherwise it had a beautiful buffer overflow).

And of course we need to do this both for the current_dive and the
displayed_dive.

Fixes #616

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>