App: Add the definition of the text domain

This adds the definition of the text domain used by gettext to
translate the strings of the user interface. gettext will search for
a package named "fractal" in the directory "./fractal-gtk/po". Thus
MO files containing the translations must be found in
"./fractal-gtk/po/[LANG]/LC_MESSAGES/fractal.mo".

Closes https://gitlab.gnome.org/World/fractal/issues/89

fractal-gtk/src/app/mod.rs

@@ -1,7 +1,9 @@
 extern crate gtk;
 extern crate gdk;
+extern crate gettextrs;
 
 use self::gtk::prelude::*;
+use self::gettextrs::{setlocale, LocaleCategory, bindtextdomain, textdomain};
 use std::env;
 use std::sync::{Arc, Mutex};
 use std::sync::mpsc::channel;
@@ -79,6 +81,12 @@ impl App {
             let bk = Backend::new(tx);
             let apptx = bk.run();
 
+            // Set up the textdomain for gettext
+            setlocale(LocaleCategory::LcAll, "");
+            bindtextdomain("fractal", "./fractal-gtk/po");
+            textdomain("fractal");
+
+
             let ui = uibuilder::UI::new();
             let window: gtk::Window = ui.builder
                 .get_object("main_window")