TreeTableView: must select leaf row on click into indentation region

.. but doesn't (after row was re-used?). To reproduce, compile and run the example below

- click into leading empty space on any of visible leafs (root or expanded child)
- expected and actual: leaf selected
- expand a collapsed child, click into leading empty space of any of its leaf children
- expected: child leaf selected
- actual: child leaf not selected

The technical issue is a mixture:
- TreeTableRowSkin manages the disclosure node, seems to try to remove it if not needed but fails (when/why exactly?). At least it seems to toggle the disclosure's visibility reliably
- TreeTableCellBehavior does the hit-detection on the disclosure node if added to the row

Seems to be virulent since ages (checked against fx11 and fx8). Priority set to 3 because it's an UX issue and nothing an application developer can workaround.

A quick fix is to let the hit-detection check both existence and visibility. A cleaner fix (off scope for this, extracted to ??) would cleanup the missing removal (and possibly other layout issues around graphic/indentation in Tree/Table). A real fix might also cleanup the smeared responsibility across several collaborators)

The example (modified from https://stackoverflow.com/q/63964556/203657):

public class TreeTableLeafSelectionWithMouseBug extends Application {

    @Override
    public void start(Stage primaryStage) {
        // TreeTableView
        TreeTableView<String> treeTableView = new TreeTableView<>(new TreeItem<>("Root"));
        // column
        TreeTableColumn<String, String> primaryColumn = new TreeTableColumn<>("Col1");
        primaryColumn.setCellValueFactory(call -> new ReadOnlyStringWrapper(call.getValue().getValue()));
        treeTableView.getColumns().add(primaryColumn);

        fillTree(treeTableView.getRoot());

        HBox content = new HBox(10, treeTableView);
        Scene scene = new Scene(content);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    protected void fillTree(TreeItem<String> root) {
        root.setExpanded(true);
        root.getChildren().add(0, new TreeItem<>("leafChild"));
        for (int i = 0; i < 10; i++) {
            TreeItem<String> newChild = new TreeItem<>("child " + i);
            if (i == 0) newChild.setExpanded(true);
            root.getChildren().add(newChild);
            for (int j = 0; j < 3; j++) {
                TreeItem<String> newChild2 = new TreeItem<>(i + " grandChild " + j);
                newChild.getChildren().add(newChild2);
            }
        }
    }

    public static void main(String[] args) {
        launch(args);
    }
}