Summary:
Even though the Slide effect has a KCM, it's not possible to configure
it [slide effect] from the virtual desktops KCM.
This change addresses the problem above.
### Before
{F5912774, layout=center, size=full}
### After
{F5912775, layout=center, size=full}
BUG: 395377
Reviewers: #kwin, davidedmundson
Reviewed By: #kwin, davidedmundson
Subscribers: davidedmundson, kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D13544
586460dbfc