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