Tree/Item/ModificationEvent: must fully support list changes

missing support:

- changes with subchanged (f.i. discontinous removes/adds)
- permutations

Without, there's no way interested client code can keep state related to current locations of items. Examples of how internal code suffers from the omissing is selectedIndices/selectedItems of TreeView. (Which currently might not be claringly obvious because all internal implementations of selection models are incorrect in respect to multiple subchanges, tree selection bugs show the same symptoms). While the list-based models can be implemented correctly (see f.i. IndicesList/IndexedItems in my git), the tree-based can't.

Some pseudo-code snippets:

    // expect a single notification because ...
    @Test
    public void testChildEventDiscontinousRemoved() {
        IntegerProperty p = new SimpleIntegerProperty(0);
        EventHandler<TreeModificationEvent> l = e -> {
            assertTrue(e.wasRemoved());
            p.set(p.get() + 1);
        };
        treeItem.addEventHandler(TreeItem.childrenModificationEvent(), l);
        children.removeAll(children.get(2), children.get(5));
        assertEquals("received singe removed", 1, p.get());
    }

    // without single notification can't safely update stored indices
    @Test
    public void testChildEventSubChanges() {
        // simulate external storage of last index
        IntegerProperty p = new SimpleIntegerProperty(children.size()-1);
        EventHandler<TreeModificationEvent> l = e -> {
            assertTrue("sanity: got a removed", e.wasRemoved());
            // adjust index: fine for single subChange
            p.set(p.get() - e.getRemovedSize());
            // this must be valid but isn't because it's the first
            // of a sequence of separate notifications
            children.get(p.get());
        };
        treeItem.addEventHandler(TreeItem.childrenModificationEvent(), l);
        children.removeAll(children.get(2), children.get(5));
    }

    // no way to update index-based state after sorting
    @Test
    public void testChildEventPermutated() {
        EventHandler<TreeModificationEvent> l = e -> {
            assertTrue("sanity: got a permutated", e.wasPermutated());
            // now how to get the permutation indices?
        };
        treeItem.addEventHandler(TreeItem.childrenModificationEvent(), l);
        Comparator<TreeItem<String>> c = (TreeItem<String> o1, TreeItem<String> o2)
                -> o1.getValue().compareTo(o2.getValue());
        children.sort((Comparator<? super TreeItem>) c);
    }