Replace Intltool with GNU Gettext

GNU Gettext 0.19.6 can translate appdata xml files and it is actively
maintained which Intltool isn't.

xgettext is unable to parse G_GNU_PRINTF annotations so add all the
printf style functions from glib and gio with --flag. This ensures that
when these functions are called with string literals which do not
contain ‘%’ specifiers xgettext inserts ‘c-format’ comments into the pot
file for msgfmt -c checks.

https://bugzilla.gnome.org/show_bug.cgi?id=769623