region: build IBus setup app filename only from the engine prefix

There are 84 m17n engines in the whitelist and they all use
ibus-setup-m17n.  To reduce the number of .desktop files, this
patch uses the prefix before the first ":" to construct the .desktop
filename and set IBUS_ENGINE_NAME envvar to let the setup app know
the actual engine ID.

https://bugzilla.gnome.org/show_bug.cgi?id=684935