then, use that to change how it works for the scheme mode (and also another variation for the REPL to
cope with the prompt)
I spent a while trying to make this work at the keymap% level (ie putting different keybindings for "home"
and "c:a" into different keymaps) but this just turned out to be far too confusing and fragile, so went
with this alternative (one keybinding, but that delegates to an overridable method)
closes PR 11446
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.
The bug was that the planet log message registration was happening
on drracket's thread instead of on the user's thread, so it was
using the wrong keys in the ephemerons.
language dialog, not the #lang htdp/* variants), teachpacks are put into the
initial REPL (instead of just having the language primitives).
closes PR 11160
