Gets the Pango.Layout used to display the entry. The layout is useful to
e.g. convert text positions to pixel positions, in combination with
get_layout_offsets().
Keep in mind that the layout text may contain a preedit string, so
layout_index_to_text_index() and text_index_to_layout_index() are needed
to convert byte indices in the layout to byte indices in the entry
contents.