#9 Expression Input validation

[Tom Downey]

When I did the Swing conversion, I commented out the
character input validation code in ExpressionInput.
Things get checked by the parser eventually, so this
seems mostly OK. But we may want to add back in some
chacking while the user is typing, possibly using a
JFormattedTextField to do the checking.

[Tom Downey]

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Same issue for VariableInput