Make the animation slider more granular

Summary:
Between default animation speed and slower/faster, there is a huge change.
Making the slider change from 0.5 instead of one,
makes the speed of animation change in much smaller changes

Reviewers: Plasma, VDG, ngraham, davidedmundson, broulik

Reviewed By: Plasma, VDG, ngraham, davidedmundson, broulik

Subscribers: broulik, ngraham, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28053