[effects/slide] Add 'Duration' option
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