Look for a property at click position.
POS may be either a buffer position or a click position like
those returned from `event-start'. If the click position is on
a string, the text property PROPERTY is examined.
If this is nil or the click is not on a string, then
the corresponding buffer position is searched for PROPERTY.
If PROPERTY is encountered in one of those places,
its value is returned. (fn POS PROPERTY)