dynamic dispatch refactored

rename struct server -> struct ircconn

remove all references to fe_* from server.c, other than ctor/dtor

remove all mentions of struct session from server.c

remove gui_ulist_style option

this is a suboptimal solution, but a more advanced theming system should be worked on later

Fixes: https://todo.amehut.dev/~aleteoryx/sexchat/12

hexchat-2.16.2 tree