[effects/slide] Add "Slide desktop background" option

Summary:
Some people may not like the sliding of desktop background. Add
corresponding option to disable the sliding of desktop background.

By disabling the sliding of desktop background and docks, one can
get old slide effect.

{F5912713, layout=center, size=full}

Test Plan:
* Unchecked "Slide desktop background" checkbox, switched desktop;
* Checked "Slide desktop background" checkbox, switched desktop.

Reviewers: #kwin, #plasma, #vdg, mart

Reviewed By: #kwin, #plasma, #vdg, mart

Subscribers: romangg, abetts, ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13542