Minor changes so the source could compile and fix for #738448

- empathy-contact-chooser.c was raising a warning/error due to an uninitialized variable
- empathy-theme-adium.c was invoking deprecated functions and structs from Webkit3
- gtk config from empathy-preferences.ui was making preferences dialog strangely wide

(I'm sending this request mostly because some people from my CS course were trying to compile Empathy from source and were having a hard time with it. Cheers.)