Connect Application::screensCreated to WaylandServer::initOutputs

Summary:
Instead of having the Application invoke initOutputs after creating
the Screens, we can just connect to the signal emitted there.

This allows to make initOutputs a private, WaylandServer internal
method.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1482