Support custom color scheme for Wayland windows

Summary:
On Wayland the QtExtendedSurface is used to pass the path to the color
scheme using the generic window property. To support this ShellClient
installs an event filter on the extended surface and listens for dynamic
property change events.

As the color scheme is KDE specific anyway it is not a problem that a Qt
specific interface is being used. For other toolkits just passing the
name to KDE's color scheme isn't sufficient anyway. If at some point we
find a way to support this for e.g. GTK+ we would need to come up with a
dedicated interface.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D3047