GtkSourceIter, the come back

Add GtkTextIter functions to implement custom word boundaries. It'll be
used in gtksourceview.c. It's better to have a separate file with those
functions so the file can easily be copied in applications, for example
gnome-builder (to implement word movements for the Vim mode). It maybe
makes sense to publish those functions as public, but as a first step
let's keep that private.