Wide character input & display fixes

This adds a dependency on Glibmm and switches various internal users to `gunichar` & `Glib::ustring`. These fixes various bugs in both display (e.g. body line wrapping, header wrapping, row highlighting etc.) as well as input (status line Unicode input (#102), wide character keypresses etc.).

This has only seen some very basic testing on my setup and I'm not at all familiar with lumail, so I wouldn't be suprised if it's full of bugs. Please inspect closely :)

It's also incomplete, in the sense that not all `std::string` users have been converted or even checked upon. `src/screen.cc` in particular has a few more users.

(it's 2013 and we still need special libraries to deal with it…)