Fix crash if we have no divecomputer information

The string we print is lame, but it keeps things consistent (and prevents
us from dereferencing functions in uninitialized objects).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Don't print temperatures that are below 200K

That threshold is of course ridiculous and arbitrary - but it seems like a
good assumption to make that anything below that is DEFINITELY not valid
data.

Because of the way the scene grows automatically in Qt, printing these
texts would squish the profile into one thin line.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Modify selection changed slot to deal with trips correctly

If a user clicks on a trip, all the dives in a trip should be selected.
But if a user selects a range of dives that happens to have a trip header
in it, then only the range of dives should be selected (the trip header is
marked as 'selected' for visual consistency, even though not all dives in
this trip are selected).

This also changes the code to scrollTo the first selected dive instead of
just expanding the parent. This seems to give us a more pleasant visual
appearance (trying to keep the selected dive centered in the dive list)
and as a side effect no longer hides the first dive trip at program start
(before this change the first dive in the first trip would be the top
entry in the dive list, with its trip just out of sight above).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Allow walking the dive list with the cursor keys

Figure out what is our first selected element (in case we start out from a
multiple selection) and then move to the next logical element. So the code
traverses an expanded tree (from a trip 'down' to its first dive or 'up'
to the last dive of the previous trip - and similar from a first dive in a
trip 'up' to its trip and from a last dive in a trip 'down' to the next
trip.

This does not take 'shift-cursor-up/down' into account (i.e. manual
selection extension). Instead with just cursor up and down a single dive
(or single trip) is selected.

My guess is that the code will make someone's eyes bleed. Be warned.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Merge branch 'Qt-selection' into Qt