[wayland] Properly implement minimize of ShellClient

Summary:
We need to call Workspace::clientHidden when minimizing. Otherwise
Workspace doesn't update the active client and the now minimized
window is still active.

BUG: 366634

Test Plan:
Test case works now and also tested in nested setup that
active window changes.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2410