Summary:
A small helper class is added which manages inhibiting idle for the
ShellClients. So far only very basic functionality is added. That is
only the inhibition on the Surface is followed. It is not yet checked
whether the ShellClient is visible at all. That needs some changes in
ShellClient.

BUG: 385956
FIXED-IN: 5.12

Test Plan: New test case passes

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8856