Summary:
At the moment, there is no way to tweak duration of the slide animation.
This change adds a configuration module so it is possible to change
the duration.
Test Plan:
* enable virtual desktops
* go to `System Settings > Desktop Behaviour > Desktop Effects`
and select Slide effect under "Virtual Desktop Switching Animation"
* click settings/options button and change duration
Reviewers: #kwin, #plasma, graesslin
Reviewed By: #kwin, #plasma, graesslin
Subscribers: graesslin, plasma-devel, kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D9382
Vlad Zagorodniy