[wayland] Support for hideClient in ShellClient

Summary:
This implements the hideClient for ShellClient which was previously
not implmented. Also autotest for same is added.

Test Plan: ran autotest

Reviewers: graesslin, Plasma

Reviewed By: graesslin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D3022