Summary:
So far we distinguished `Layout.Container` from `Layout.Vertical`, but they did almost exactly the same, so figured to unify them, so smaller API ftw :) Normal containers put children vertical, and if you want to use horizontal, use Layout.Horzontal
Also simplified code in Layout file a little bit.
Fixed issue I split container where the main container didn't go "underneath" the fixed container
Reviewed By: cekkaewnumchai
Differential Revision: D24502547
fbshipit-source-id: 517db3692749e670cda8f0cd7cb1c807df818b4d
30f5f0b59a