A function that will be called to provide the contents of the selection.
If multiple types of data were advertised, the requested type can
be determined from the info parameter or by checking the target field
of selection_data. If the data could successfully be converted into
then it should be stored into the selection_data object by
calling SelectionData.set (or related functions such
as SelectionData.setText). If no data is set, the requestor
will be informed that the attempt to get the data failed.
A function that will be called to provide the contents of the selection. If multiple types of data were advertised, the requested type can be determined from the info parameter or by checking the target field of selection_data. If the data could successfully be converted into then it should be stored into the selection_data object by calling SelectionData.set (or related functions such as SelectionData.setText). If no data is set, the requestor will be informed that the attempt to get the data failed.