If set, this function is called before any node is selected or unselected,
giving some control over which nodes are selected. The select function
should return %TRUE if the state of the node may be toggled, and %FALSE
if the state of the node should be left unchanged.
Sets the selection function.
If set, this function is called before any node is selected or unselected, giving some control over which nodes are selected. The select function should return %TRUE if the state of the node may be toggled, and %FALSE if the state of the node should be left unchanged.