shell/layout: Add implementation for the width param

This adds support for passing by the width argument such that users can
specify the width dimensions of the split window. Given that we might
another previous active window we also need to split that accordingly
depending on the orientation.

Bug-AGL: SPEC-4839
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I39239c1af0358eaa671146d0f36a3a334e945067