Add irc_notice_pos setting

This removes gui_tab_notices and allows for the option to always send to front which previously required a very popular script https://docs.hexchat.org/en/latest/tips.html#how-to-make-notices-show-up-in-a-consistent-location .

0 = automatic, the default same behaviour as now.
1 = in their own tab, (snotices) and (notices), previously gui_tab_notices
2 = always in the front tab