Preparation for color printing

this patch adds a 3rd column to the media array of rgba so we now have one
each for screen, b/w & color printing. I have defaulted to using the
same color printing & screen, but this can be altered anytime.

I have checked that the application still compiles and prints.  The print
out (colour option selected) shows the deco ceiling steps in pink but
everything else appears grey scale. Further work will be required to apply
the colours to the print out, although I'm not yet sure what that involves.

Signed-off-by: Amit Chaudhuri <amit.k.chaudhuri@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Add all dive computers to the device_info_list

Calling set_dc_nickname() only added a new entry if there was a conflict
with an existing one - which works fine if we are reading a file saved by
Subsurface, as there the device_info_list is populated at parse time, but
fails for data imported from other sources.

Fixes #74

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>