#151 Edit history in DefinitionsPane

[Charles Reis]

Since the interactions history is essentially Java code
anyway, it would be convenient to edit it in a
DefinitionsPane rather than its own window. This would
require us to remember that it needs to be saved with a
".hist" extension rather than ".java", but the
indenting and highlighting should be the same as a Java
document. (This would be a first step towards
supporting non-Java files in the DefinitionsPane area,
such as plain text.)

Note this would also eliminate the odd bug that
JTextAreas on OS X use Ctrl+X, etc for cut, copy,
paste, rather than Command+X, etc.