Conversion to gettext to allow localization

This is just the first step - convert the string literals, try to catch
all the places where this isn't possible and the program needs to convert
string constants at runtime (those are the N_ macros).

Add a very rough first German localization so I can at least test what I
have done. Seriously, I have never used a localized OS, so I am certain
that I have many of the 'standard' translations wrong. Someone please take
over :-)

Major issues with this:

- right now it hardcodes the search path for the message catalog to be
  ./locale - that's of course bogus, but it works well while doing initial
  testing. Once the tooling support is there we just should use the OS
  default.

- even though de_DE defaults to ISO-8859-15 (or ISO-8859-1 - the internets
  can't seem to agree) I went with UTF-8 as that is what Gtk appears to
  want to use internally. ISO-8859-15 encoded .mo files create funny
  looking artefacts instead of Umlaute.

- no support at all in the Makefile - I was hoping someone with more
  experience in how to best set this up would contribute a good set of
  Makefile rules - likely this will help fix the first issue in that it
  will also install the .mo file(s) in the correct place(s)

  For now simply run

  msgfmt -c -o subsurface.mo deutsch.po

  to create the subsurface.mo file and then move it to
  ./locale/de_DE.UTF-8/LC_MESSAGES/subsurface.mo

  If you make changes to the sources and need to add new strings to be
  translated, this is what seems to work (again, should be tooled through
  the Makefile):

  xgettext -o subsurface-new.pot -s -k_ -kN_ --add-comments="++GETTEXT" *.c
  msgmerge -s -U po/deutsch.po subsurface-new.pot

  If you do this PLEASE do one commit that just has the new msgid as
  changes in line numbers create a TON of diff-noise. Do changes to
  translations in a SEPARATE commit.

- no testing at all on Windows or Mac
  It builds on Windows :-)

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

file.c

@@ -4,6 +4,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include <glib/gi18n.h>
 
 #include "dive.h"
 #include "file.h"
@@ -71,7 +72,7 @@ static void suunto_read(struct zip_file *file, GError **error)
 		size = read * 3 / 2;
 		mem = realloc(mem, size);
 	}
-	parse_xml_buffer("SDE file", mem, read, error, FALSE);
+	parse_xml_buffer(_("SDE file"), mem, read, error, FALSE);
 	free(mem);
 }
 #endif
@@ -258,11 +259,11 @@ void parse_file(const char *filename, GError **error, gboolean possible_default_
 		if (default_filename && ! strcmp(filename, default_filename))
 			return;
 
-		g_warning("Failed to read '%s'.\n", filename);
+		g_warning(_("Failed to read '%s'.\n"), filename);
 		if (error) {
 			*error = g_error_new(g_quark_from_string("subsurface"),
 					     DIVE_ERROR_PARSE,
-					     "Failed to read '%s'",
+					     _("Failed to read '%s'"),
 					     filename);
 		}
 		return;