Add gimp_text_buffer_get_iter_at_index() which does the reverse thing
than the already existing function gimp_text_buffer_get_iter_index().
Use the new function when cursor-navigation lines. Add "gboolean
layout_index" to both functions, which if TRUE indicates that the
passed in/out index is an index into the PangoLayout's content rather
than the text buffer's. When dealing with layout indices, take into
account the additional characters we insert into the serialized markup
(and thus the layout) for each character that is tagged with spacing.