Sirc(fix): Convert IRCv3 received server-time to local timezone (#379)

[Exagone313]

Closes #379

I'm not sure if g_time_zone_new_local() could be called more globally instead of on each message.

I also wonder if the timezone support handles daylight saving (if you'd keep Srain open when the timezone offset changes, would it keep outputting the right date if the GTimeZone object doesn't change?), though it would be a minor issue if it doesn't.