[plt-dev] language dialog, some minor changes & bugfixes

What if the default where

        ---------------------------
 #lang |click me to pick a language|
        ---------------------------

?

An experienced user could delete the box and type "scheme" or
"typed-scheme" or whatever. A student could click the box and get a
list of popular choices, including the HtDP languages. And if someone
just picks "scheme", then they've picked the right thing.

This interface both help people pick a language and reinforce the idea
that the "#lang" line determines the language.