[gtk3] Mnemonics used sporadically

Properties -> General, long press the Alt key. Out of the twenty-ish options, only two receive an underline for one of its letters. Mnemonics aren't used too much on other tabs either.

(By the way, these mnemonics don't work for me at all in gnome-terminal. I guess I should rather be taking a look at that, instead of reporting the same against guake :D )