webadmin: Added HorizontalSplitTable widget

This widget comprises two tables, separated horizontally. It simulates
import/export situations, where entries from one table are to be moved
to another. This is done by selecting the proper entries in one of the
tables, then clicking the corresponding arrow button to add them to
the other table.

Change-Id: I47b79ed09b69980ef763fecade0940bab0d517ec
Signed-off-by: Lior Vernia <lvernia@redhat.com>