add dive - broken WIP for Tomaz

[Dirk Hohndel]

As the subject indicates, this is primarily intended for Tomaz to see
what damage I've been trying to do ^W^W^W^W^W^W I've been working on.

The idea is to use the planner to also simply add dives. And of course
to then be able to edit / change dives like that with that same
graphical editor later.

Some of this works already, some of this has just been started.

The biggest issue is introduced at the end where I switch back to having
the maintab widget shown (that one is much more useful when adding a
dive). But of course that widget has an existing edit mode that makes
very different assumptions about what 'OK' and 'Undo' might mean.

As the last commit indicates, this means that we need a third editMode
(ADD) in which OK and Undo do the right thing and consolidate the latest
two dive structures into one and add this consistently to the dive list.

We also need to add a field to edit the dive date and time as well as a
couple of other data points we might want to be able to enter
(temperatures for example).

Anyway, this is very much WIP and just intended for feedback.

testForTomaz branch on git.hohndel.org (not on github)

/D