We used to implement it as a list box containing several drawing area.
To make reflow implementation simpler, this consolidate all the
rendering logic into a single place.

https://bugzilla.gnome.org/show_bug.cgi?id=740260

Conflicts:
	src/window.js

To support Unicode 7.0.0, import necessary modules from Gnulib, where
more up-to-date libunistring code resides.  Those modules won't be
linked to libgc, once libunistring 0.9.5 is released.