gtk-selection-data-set (selection type format data length)

selection -- a gtk-selection-data instance
type -- a gdk-atom as a string with the type of selection data
format -- an integer with the format, number of bits in a unit
data -- a pointer to the data, will be copied
length -- an integer with the length of the data

Stores new data into a gtk-selection-data instance. Should only be called from a selection handler callback. Zero-terminates the stored data.