TreeSelectionFunc  
    typealias TreeSelectionFunc = (selection: TreeSelection, model: TreeModel, path: TreePath, pathCurrentlySelected: Boolean) -> Boolean(source)
A function used by gtk_tree_selection_set_select_function() to filter whether or not a row may be selected. It is called whenever a row's state might change.
A return value of true indicates to @selection that it is okay to change the selection.
- param - selectionA- GtkTreeSelection
- param - modelA- GtkTreeModelbeing viewed
- param - pathThe- GtkTreePathof the row in question
- param - pathCurrentlySelectedtrue, if the path is currently selected
- return true, if the selection state of the row can be toggled