add timestamp as a field in sign-level information

[kchall]

Add "timestamp" as a field within "sign-level" information that can then also be displayed in the "Corpus" window and sorted by, along with Entry ID, Gloss, Lemma, and ID-Gloss. This is in preparation for interfacing with ELAN, where we might have sequentially ordered signs instead of just dictionaries. Eventually we'll want to consider things like formatting, automatic filling-in, etc., but for now, we just need the architecture in place.