webadmin: AddRemoveRowWidget to enable/disable rows

Added the ability for AddRemoveRowWidget to change whether individual
rows are enabled/disabled according to the underlying model's
changeability.

This obviated the corresponding implementation in KeyValueWidget
(which will now benefit from AddRemoveRowWidget's capabilities by
extending it), and in various other places where this behavior had to
be implemented manually.

Change-Id: Ia9a6265b5c0eb27c3b99b98d88986762249acdbe
Signed-off-by: Lior Vernia <lvernia@redhat.com>