Programmatic-end for parsing RSel expression text and rebuilding the expression in the user interface.

NOTE: This is a slimmed down version of the full RSel Expression Editor. The RSelExprParser.updateExpression(exprText) method provided in this script can be used for parsing saved expression text. The match with RSel's original expression text must be fairly close and it does not allow much room for human error. See the script for the full Expression Editor for a more flexible implementation, though it is not provided in the form of a library script.