https://bugzilla.gnome.org/show_bug.cgi?id=730745

Mostly for debugging pruposes, but use it for printing CSS errors in
GtkCssProvider, too.

The parser has the file available.

Most of these are forgotten :'s and similar details
which gtk-doc now warns about.

Instead of path, line and position, emit the section the error happened
in. This has a lot more information to consume.

This struct keeps track of an area of text in a CSS file and uses it
when specifying information. Also, the cssprovider keeps track of
sections when parsing a file.