webadmin: KeyValueWidget to inherit AddRemoveRowWidget

Refactored KeyValueWidget to inherit from AddRemoveRowWidget, in order
to promote a uniform look and feel throughout the project as well as
code reuse.

Change-Id: I5c633210139c501e6709e3c68c1d70af5bdfe111
Signed-off-by: Lior Vernia <lvernia@redhat.com>