Chat log message sender ID is actually a timestamp.

[PunishedPineapple]

What is listed as sender ID in XivChatEntry and ChatGui appears to actually be a Unix timestamp. This PR marks the property as obsolete with a warning message to this effect, and changes the XML comments and internal uses.

I'm not sure how this should be fixed in the plugin-visible API; whether new events/delegates should be added with the correct parameter names, or wait for the next plugin API bump, and make it a hard break (rename parameters, make them refs, and rename the property in the struct).

Anyway, I've never seen an ID of any kind in that field, and setting it to a timestamp when printing a message shows the expected time in the chat log. Given that the chat log stuff is used by a ton of plugins, though, I'd feel better with a second eye on this. It feels a bit wrong countering RE work that's probably already had a lot of knowlegable people look at it.