Accept a :-separated list of module names in GTK_IM_MODULE and
the corresponding setting, to deal a bit better with broken
situations.
https://bugzilla.gnome.org/show_bug.cgi?id=603559
Patch by Akira Tagoh, backported from GTK+ 3. The backport
is required because GTK+ 2 and 3 are listening to the same env
vars and settings for immodules.