webadmin: Introducing AddRemoveRowWidget

This is a reusable widget used to display a list of other widgets,
backed by a ListModel of values. Entries may be removed or added using
minus/plus buttons. A new entry is added by activating a semi-disabled
"ghost" entry.

Change-Id: I15958e4b7c1fcf0ec08ea35eb32c690964f2a366
Bug-Url: https://bugzilla.redhat.com/1003779
Signed-off-by: Lior Vernia <lvernia@redhat.com>