[gtk3] Default encoding changed

VTE changed its encoding at one point. It used to default to the locale's one, now it defaults to UTF-8.

Since Guake doesn't have an option for the encoding and doesn't set it, this results in a visible change compared to gtk2-based Guake for those who don't use UTF-8.

In the gtk2 version, Guake started the terminal with the locale's encoding, and hence provided a consistent, working (albeit obsolete) experience to its users.

With the gtk3 version, the terminal's actual behavior is UTF-8, whereas the locale describes a legacy encoding towards the applications running inside, resulting in all the non-ASCII characters being broken.

In order to preserve the previous behavior and provide a consistent, working experience for legacy users, I recommend Guake to execute the python equivalent to `vte_terminal_set_encoding(..., nl_langinfo(CODESET), ...)` in its terminals.