Undo: Implement undo of dive site name editing

Implement an undo command that edits the name of a dive site.
Connect it to the dive site table, so that names can be edited
directly in the table.

Send signals on undo / redo so that the dive site table and
the dive site edit widget can be updated.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/command.h

@@ -41,6 +41,7 @@ void mergeDives(const QVector <dive *> &dives);
 // 3) Dive-site related commands
 
 void deleteDiveSites(const QVector <dive_site *> &sites);
+void editDiveSiteName(dive_site *ds, const QString &value);
 
 } // namespace Command