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