deco code rewrite

[Dirk Hohndel]

I pushed out some code to improve deco handling and add NDL support
    
This changes the code that was recently introduced to deal with deco
ceilings. Instead of handling these through events we now store the
ceiling (which in reality is the deepest deco stop with all known dive
computers) and the stop time at that ceiling in the samples.

This also adds support for NDL (non stop dive limit) which both dive
computers that at this time give us ceiling / deco information appear to
give us as well (when the diver isn't in deco).

If the mouse hovers over the profile we now add support for displaying
the NDL, the current deco obligation and (if we are able to tell from
the data) whether we are at a safety stop.

The XML file format stayed as closely to the existing format as
possible, so we once again only store information that has changed...
Sorry, if you are one of the two people who tested the code that was in
git for the last week or so... I simply threw that away and the events
that were created are no longer usefully supported...

Big thanks to Linus for helping me write sane code for this - I was deep
on my way into a rat hole... (sorry, forgot to add that to the commit
message).

Please give it a test. Especially, of course, if you have one of the two
dive computers that at this point should provide the necessary info.
a) the OSTC (this requires the latest trunk of libdivecomputer which
adds the deco / ndl support for the OSTC)

Jan and I have one of those. Anyone else?

a) the Uemis SDA (this no longer requires the latest libdivecomputer -
as Linus had pointed out, it was really silly to have that dependency)

Bearsh, Linus and I. Anyone else?

But I want to make sure that I didn't break anything else by mistake, so
a general round of testing might be a good idea as well.

/D