i18n: mark strings in code for translation

Marks all string in the code to be translated with gettext macros.
POTFILES.in is updated to only contain source files with translatable strings.
po/README instructions are updated so that xgettext finds all the used macros.

C_() is used to give context to a translatable string.

Issue: #78928
Change-Id: I27bd91ff12a9913ac0d41842e449daf73197d96b
Signed-off-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>
diff --git a/src/utils/menus.cpp b/src/utils/menus.cpp
index ffb1ccc..ab498c8 100644
--- a/src/utils/menus.cpp
+++ b/src/utils/menus.cpp
@@ -30,6 +30,7 @@
 
 #include "menus.h"
 
+#include <glib/gi18n.h>
 #include <contactmethod.h>
 #include "../contactpopover.h"
 
@@ -63,7 +64,7 @@
 {
     g_return_val_if_fail(cm, NULL);
 
-    auto add_to = gtk_menu_item_new_with_mnemonic("_Add to contact");
+    auto add_to = gtk_menu_item_new_with_mnemonic(_("_Add to contact"));
 
     /* save the parent widget in the item object, so we can retrieve
      * it in the callback */