data: port from intltool to pure gettext

Since gettext 0.19.7, released in 2015, more or less all uses of intltool have been subsumed by gettext itself. The use of intltool is a legacy of the days before gettext had proper support.

Gnome has a policy document describing the transition.

https://wiki.gnome.org/MigratingFromIntltoolToGettext

Along the way, stop attempting to translate the ibus engine files. Rewrite the pattern rules to allow directly replacing @X@ patterns in them, without going through two build stages. The intltool build stage was a waste of time because nothing was marked for translation, and nothing was translated -- it is apparently not actually expected to be user interfacing.

Most likely, similar to gsettings, they were only .in.in files as a workaround for the fact that the intltool rules "grabbed" ahold of all *.xml.in -> *.xml patterns and wanted to do something with them, even if that amounts to copying them unmodified.