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