text: Add the coords_to_position() method

The reverse of position_to_coords().

While providing documentation on how to implement it using the
PangoLayout API, I realized that the verbosity of it all, plus the usage
of the Pango API, was not worth it, and decided to expose the method we
are using internally.

doc/reference/clutter/clutter-sections.txt

@@ -1966,6 +1966,7 @@ clutter_text_get_cursor_size
 
 <SUBSECTION>
 clutter_text_activate
+clutter_text_coords_to_position
 clutter_text_position_to_coords
 clutter_text_set_preedit_string
 clutter_text_get_layout_offsets