Port away from deprecated QStyle::SH_Widget_Animate
Summary:
Use the new QStyle::SH_Widget_Animation_Duration instead,
which allows us to not manually hardcode the duration.
We still use 1 if animations are disabled,
otherwise the scroll would not work at all.
See also D5883 for some background.
Test Plan: Played with different values in the Animation tab of the Breeze config dialog.
Reviewers: Dolphin, broulik
Reviewed By: broulik
Subscribers: broulik
Differential Revision: https://phabricator.kde.org/D10005