[effects/slide] Handle moving clients

Summary:
Currently, slide effect doesn't handle a case when there
is a moving client. This results in having window jumps.
This commit fixes it by fixing position of the moving client
during switching to another desktop.

Test Plan: * send window one desktop to the left/right using shortcuts

Reviewers: #kwin, #plasma, graesslin

Reviewed By: #kwin, #plasma, graesslin

Subscribers: graesslin, plasma-devel, kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9487