x and y. Buffer coordinates are coordinates for 
the entire buffer, not just the currently-displayed portion.  
If you have coordinates from an event, you have to convert 
those to buffer coordinates with 
gtkTextViewWindowToBufferCoords.gtkTextViewGetIterAtPosition(object, x, y)objectxyiterGtkTextIter. [  out  ]trailingNULL, location to store an integer indicating where
in the grapheme the user clicked. It will either be
zero, or the number of characters in the grapheme.
0 represents the trailing edge of the grapheme. [  out  ][  allow-none  ]gtkTextViewGetIterAtLocation,
which returns cursor locations, i.e. positions between
characters.
  Since 2.6