webadmin: introduce abstract split table

Add SplitTable class which holds the logic and view that are relevant
for both horizontal and vertical split tables.
HorizontalSplitTable class now extends SplitTable and holds the data
which is relevant only for horizontal split tables.

Change-Id: Id7016991693956369df6ced8322dc0836e9437e4
Signed-off-by: Arik Hadas <ahadas@redhat.com>