i18n: a locale per app, catalogs of messages, mirrored stacks

i18n::I18n is a gpui global holding the shown locale and one catalog of
messages per locale. text fills each {name} from its arguments; a
missing key, argument or catalog fails, and every catalog holds the
first one's keys. Locale writes numbers with its separators and dates
in CLDR's short numeric form; Hebrew replaces Arabic, whose Egyptian
digits Ely does not write. Direction::row mirrors a stack and
Direction::align sets text against the reading side. The gallery's
chapter 40 shows the messages in seven locales, every locale's number
and date, and a file list that mirrors under Hebrew.

examples/gallery/main.rs

@@ -99,6 +99,7 @@ fn main() -> Result<()> {
         .run(move |cx: &mut App| {
             ely_gpui_component::init(cx);
             pages::bind_keys(cx);
+            pages::install_i18n(cx);
             cx.bind_keys([KeyBinding::new("cmd-q", Quit, None)]);
             cx.on_action(|_: &Quit, cx| cx.quit());
             menus(cx);