textiter: set/get boundary type

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

This commit is just an idea, it's far from finished.

https://bugzilla.gnome.org/show_bug.cgi?id=562767
Add GtkTextBuffer::move_iter vfunc