textiter: set/get boundary type

This way, all the words/sentences-related GtkTextIter functions can be
reused for custom boundaries, and still be able to use the Pango
boundaries for e.g. spell checkers.

With the GtkTextBuffer::move_iter vfunc, all boundary types are
overridable. For example GtkSourceBuffer can override word boundaries
used for cursor movements so it better fits source code.

https://bugzilla.gnome.org/show_bug.cgi?id=562767