Introduce new pre-layout script hook

Summary:
layout.js used to be run before any containments were added. This was
broken in a697d291 which creates desktop containments for that activity
first.

However, some scripts rely on being able to alter global configuration
before any containments are loaded.

In order to allow the same functionality we need to provide a new hook
for whatever these scripts may need to do.

BUG: 371704

Test Plan: Not done yet.

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: apol, mart, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3220