Keep track of diff to initiatial state.

[andreElrico]

First of all. Thank you for ngx-treeview <3 !

In this.treeviewComponent.selection we keep track of checkedItems and uncheckedItems

{checkedItems: Array(3), uncheckedItems: Array(5)}

It would be great to have a third key changedItems:

this.treeviewComponent.selection.changedItems

This would keep track of items that have changed in comparison to the initial state!

###Anyone has a workaround or any idea how I could get the DIFF items?