#92 Warn if interactions out of sync

[Charles Reis]

If anything is entered in the Interactions Pane when the
open documents are out of sync, a warning message
should be displayed (much like in DrScheme). This
should definitely be triggered by files which have been
saved but not compiled, and probably also triggered by
modified but unsaved files. The message will only go
away after the Interactions Pane is reset (possibly
through a compile). Perhaps, unlike DrScheme, we can
only show the message once, but always have it appear
just before the prompt.

[Charles Reis]

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This is still something I'd like to see, but it's not
highest priority right now. I'm going to set it back to 5,
and we can elevate it after some of the other features are
taken care of.

[Peter Centgraf]

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Note that this feature is more important for the Eclipse plug-in
version of the interactions pane, because of incremental
compilation. It would be nice to have a lightweight "trigger"
method on the InteractionsModel to indicate that a warning
should be presented. This trigger would be called after any
operation that might cause the interactions JVM to lose sync.
There would also need to be a corresponding "clear" method,
which would indicate a manual resync had occurred. (Note
that the "clear" would only be effective after a full reset, since
any previously loaded classes could still be dirty.) The
implementation of InteractionsModel would then be able to
choose whether to always prompt, prompt only once,
automatically reset, or perform whatever other actions are
deemed appropriate.