textview: use is_word_boundary attribute for word selection

The new functions in GtkTextIter are private for now. It is almost the
same implementation as for the word start/end. The unique difference is
the use of the is_word_boundary PangoLogAttr attribute instead of
is_word_start and is_word_end.

And those private functions are used in GtkTextView for:
- move by words with Ctrl+Left, Ctrl+Right, etc.
- word selection with double click

https://bugzilla.gnome.org/show_bug.cgi?id=530405