Sync opacity from an internal QWindow to its ShellClient

Summary:
This change adds support for opacity on a KWin internal QWindow.
Calling QWindow::setOpacity syncs the value to the ShellClient
representation of the QWindow.

This is needed for fading out the OnScreenNotification on mouse
over.

Test Plan: Added auto test and manual test

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3928