Bug 1558854: Need to allow default numcolumns in lib/view.php

When we upgrade from earlier versions views need to be saved that
expect the numcolumns field to exist before we get round to dropping
it. So the View class needs to be able to define a default value for
it.

behatnotneeded - unable to test upgrades

Change-Id: Ic70add18e814975289a7145e8e91454e3699a234
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>