Remove pid changedSignal in Client::PlasmaWindow

Summary: Verified it it send before the initial_state and adjust tests and docs accordingly

Test Plan: All unit tests pass

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: graesslin, plasma-devel, #frameworks

Tags: #plasma_on_wayland, #frameworks

Differential Revision: https://phabricator.kde.org/D5887