Retrieves the iterator pointing to the character at buffer coordinates
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 [method@Gtk.TextView.window_to_buffer_coords].
Note that this is different from [method@Gtk.TextView.get_iter_at_location], which returns cursor locations, i.e. positions between characters.
if non-null, 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.
x position, in buffer coordinates
y position, in buffer coordinates
true if the position is over text