Provide a way to disable notifications for privmsgs by default

[osa1]

Currently we have no way of disabling privmsg notifications by default. Even if I do /notify off in a server tab, a privmsg tab created in that server will still have notifications on:

tiny/crates/libtiny_tui/src/tui.rs

Lines 497 to 506 in 2c723f6

	self.new_tab(
	tab_idx + 1,
	MsgSource::User {
	serv: serv.to_owned(),
	nick: nick.to_owned(),
	},
	true,
	Notifier::Messages,
	None,
	);

(Notifier::Messages part)

It might be useful to have some way of setting the default notification behavior for privmsgs.

I'm not sure how to best implement this, so marking this is "question". We could make the privmsg tabs inherit notifier of the server (like chan tabs). Is this a good idea?