New feature in midend.c which allows us to ask for the number of the
currently selected preset, if any. I've used this in the GTK front
end to have the Type menu mark the currently selected menu item.
(After considerable beating of GTK with sticks, I might add. Grr.)
Currently the same UI feature is not yet supported on Windows or
MacOS, but I hope to do those too at some point if it's feasible.

[originally from svn r7980]