[effects/slidingpopups] Tweak effect to make animation smoother and more consistent

Summary:
- Easing function for slide-in changed to InQuad
- Easing function for slide-out changed to OutQuad
- Opacity is now interpolated, too.

Reviewers: #vdg, zzag, #kwin, sefaeyeoglu, ndavis, davidedmundson

Reviewed By: #vdg, zzag, #kwin, sefaeyeoglu, ndavis, davidedmundson

Subscribers: niccolove, ngraham, abetts, davidedmundson, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18000