added spellcheck lang multiselect

this took so long, and was a good way to learn gtk. jesus though lmao

Implements: https://todo.amehut.dev/~aleteoryx/sexchat/11

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