Add a GtkEntry to allow editing of the device name

Add a GtkEntry to allow editing of the device name

Ok, so some file chooser widget with a popup dialog would have been more
professional, but I'm lazy.  Plus I suspect the popup would look
horrible when populated with /dev entries, and I don't think there is
any sane filter function.

So this works, and means that you don't *have* to recompile the whole
program just because you have your dive computer on something else than
a USB serial line.

I suspect I should save the default name as a config variable too.
Maybe a setting in the preferences dialog.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>