webadmin: Redesigned Discovery popup

It now uses HorizontalSplitTable widget, showing a provider's networks
in the top table and the networks to be imported in the bottom table.

EntityModelCellTable also required some modification to not steal the
click events from the checkbox and listbox columns.

Change-Id: I013e125c3e0f6dba91addb55727904904faf8164
Signed-off-by: Lior Vernia <lvernia@redhat.com>