queue-callback / execute callback dance; also, change the behavior
a little bit so that it works a little bit more like the rest of the
DrRacket languages; in particular, the initialization of the REPL
now only happens when a window is first opened or a new tab is first
created, but not at other times (ie not when the language changes;
when the language changes, we just keep the REPL state the same and
show a warning like before)
This change also required a change to the way the repl is initialized
and a slight change to the behavior of the first-opened method. Specifically,
it is now called in a slightly better context so that errors that
happen look like errors in the user's program. The only other use of
the first-opened method in the tree was to initialize the teachpacks
in the teaching languages and this new behavior is also an improvement
there.
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