Bug fixes in chapter 13

[dgruntz]

Fixes the treeview expand/collapse arrow bug
This commit fixes the treeview expand/collapse arrow bug as described in
https://stackoverflow.com/questions/50011276/javafx-treeview-expand-collapse-disclosure-arrow-bug

It adds a style file (tree.css) which defines the rotation angle of the arrow for both the normal and the expanded case.

Removes doubled navigate menu
Removes

    menuBar.getMenus().add(navigateMenu);

as the menu is also added with the command

    menuBar.getMenus().addAll(navigateMenu, historyMenu);

It appeared twice.

Fixes the CME exception
When an entry is removed from list historyMenu.getItems() in the loop

   for (MenuItem i : historyMenu.getItems()) {
        if (i.getId().equals(e.getUrl())) {
            historyMenu.getItems().remove(i);
        }
   }

then a CME is thrown. Iterating over a copy fixes this problem. An alternative would be to leave the loop with a break, but this only works if the menu item i appears at most once in the list.