build: Migrate from Intltool to Gettext

Recent versions of Gettext are able to translate several formats
that are used in GNOME applications. This patch migrates from
Intltool to Gettext by using meson's i18n features.

Explicit encoding has also been added to the gschema files due to
an issue in gettext[0] with default encoding in XML files.

https://bugzilla.gnome.org/show_bug.cgi?id=793087

[0] https://savannah.gnu.org/bugs/?52932