Obtains the position of the Pango.Layout used to render text in the
entry, in widget coordinates. Useful if you want to line up the text
in an entry with some other text, e.g. when using the entry to implement
editable cells in a sheet widget.
Also useful to convert mouse events into coordinates inside the
Pango.Layout, e.g. to take some action if some part of the entry text
is clicked.
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.