TreeCell: cancelEvent must return correct editing location

this is JDK-8266969 for TreeCell - to keep scope of the fix bounded by cell type :)

To summarize (treeCell specifics - copied from the blocked issued):

The invariant:

    control.edit(editingIndex);
    control.setOnEditCancel(e -> assertEquals(editingIndex, e.getIndex()));

must be kept independent of what triggered the cancel. For TreeCell, it is broken on cell re-use (detected while fixing JDK-8265210). It's caused by using the current cell location in the event:

     // TreeCell
      editingLocation = this.getTreeItem();
      fire(new XXEvent(...editingLocation...)

which on re-use is different from the one at the time of starting the edit. Tentative fix is to keep the index (treeItem) at the time of startEdit and use it in the cancelEvent.