i18n: Do not use textdomain() in libs

This makes the library the default translation domain and thus breaks the translation system for applications that use the library.

See also:

• https://developer-old.gnome.org/glib/stable/glib-I18N.html
• https://gitlab.xfce.org/xfce/libxfce4util/-/blob/0ef5af7114fd357d7cdb8b20c66f69791858a21c/libxfce4util/libxfce4util-private.c#L41

I noticed this when using the new shortcuts editor: dialogs opened after calling the editor once are no longer translated, and in general the translation system no longer works, because the domain is no longer the right one.

It's not clear that there is a common header in all these places from which to include a little _xfce_i18n_init() function, and I don't think it's worth adding an API to libxfce4util for that. After all it's only two lines to copy if needed instead of one.