Clearing dive site did not work for two reasons:
1) We didn't get a signal when editing was finished.
2) When clearing the dive site, the "add new dive site" site was set.
Thus, connect to the editingFinished signal and in
DiveLocationLineEdit::currDiveSite() return a null pointer if
the string is empty.
This means that it is not possible to have a dive site with an
empty string, but that shouldn't be a problem, right?
Fixes#2148
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
02e94de062