Undo: implement undo of dive site editing

This one is a bit more tricky. There are two modes: set dive site
and set newly created dive site. This is realized using an OO model
with derived classed. Quite convoluted - but it seems to work.

Moreover, editing a dive site is not simply setting a value,
but the list of dives in a dive site has to be kept up to date.

Finally, we have to inform the dive site list of the changed
number of dives. Therefore add a new signal diveSiteDivesChanged.
To send only one signal per dive site, hook into the undo() and
redo() functions and call the functions of the base class there.

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

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