made LaTeXiT beautiful

* special characters are escaped
* infrequent fields are commented out automatically
* there is space for users to put in LaTeX labels
* pop-up box only shows one GB4E {exe} at a time for datum (and still shows all of them for a DataList)
* made some code IGT-specific so that we can have generalized code for non-utterance/morpheme/gloss/trans text.