webadmin: Small improvements to AddRemoveRowWidget

Made the following changes in AddRemoveRowWidget:

* Added an event listener to the edited model's items changed event,
  which seems to be logic common to any usage of the widget.

* Removed the setItems() call from the flush() method, which turned
  out to be a bad idea for various reasons, and instead just update
  the current items of the model.

* Changed onAdd() and onRemove() to commit changes to the model by
  default, which seems to be useful.

* Changed onAdd() and onRemove() to only be invoked when triggered by
  a button press (e.g. not during initialization of the widget), the
  semantics of which would be much more intuitive to anyone who might
  inherit from this widget.

Change-Id: I645c6984f9bcfe6f00f8b7eb07313f4d4f3d7043
Signed-off-by: Lior Vernia <lvernia@redhat.com>