Add GPS icon to the location column for dive sites where we have GPS data

This replaces the really lame "italics text" from commit abe810ca1a29
("Mark locations that have GPS location data attached") with a marginally
less lame GPS icon.There's a reason why I am not making a living as
graphics artist. But I think this is a huge step forward from what we had
before...

The satellite.svg file is very loosely based on a different icon that I
found as public domain here http://www.clker.com/clipart-30400.html.
From that I created the PNG and then that was converted into the
GdkPixdata via gdk-pixbuf-csource; a rule for that was added to
the Makefile but commented out as I don't know if this tool will always be
available in the path. Having this icon included in the sources avoids
locating yet another icon file.

Better icons are certainly welcome!

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix suunto serial number confusion in libdivecomputer

libdivecomputer has started giving the Suunto serial numbers in a
different format, which means that we have the same device with two
different serial numbers, and then we need two different ways of turning
the numerical entity into a string.

Look at the number pattern to see figure out which version of the format
it is that libdivecomputer is reporting, and turn it back into the
original format so that we can reliably give the right string for it.
This also mean sthat the device ID stays the same regardless of
libdivecomputer version.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Improve on the "prefer downloaded" dive computer model

It used to be that when you checked the "Prefer downloaded" checkmark,
we'd throw away *any* old dive computer data.  That was good, because it
allowed us to start from a clean slate when you had some old subsurface
data with questionable dive computer data.

However, it was a bit extreme, and it's really not what you want if you
already have (good) dive computer data from other dive computers.

So this modifies the logic a bit.  Instead of throwing away all old dive
computer data, the "Prefer downloaded" checkmark now means:

 - the newly downloaded data becomes the "primary" dive computer data
   (ie the first in the list)

 - if there was old dive computer data that *could* have been from this
   new dive computer (ie it didn't have model information, or it had a
   matching model but no device ID data), we throw that away

 - but any existing dive computer data from other dive computers is left.

This seems to be much closer to what we really would want for a new
"preferred" download.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
libdivecomputer: add air temperature fixups for Suunto

libdivecomputer doesn't actually seem to support air temperature
reporting at all, but at least for Suunto dive computers the air
temperature is recorded as the temperature for the first sample.

So since we already have vendor-specific libdivecomputer hacks, let's
just add that one as a rule.  It may be that other divecomputers do this
too, so this adds it as a generic concept - it's just that right now the
flag for "air temperature in first sample" is only set for Suunto dive
computers.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>