Make selection scrolling go at 60 fps

Summary:
When you select a square with the right mouse button, and you go past the margin of the window (but where there is still space to go), okular scrolls the document so you can select more.
With this patch this automatic scrolling goes at 60 fps instead of 10, I made it so that the speed of the scroll is the same

Reviewers: Okular, aacid, sander, ngraham

Reviewed By: sander, ngraham

Subscribers: sander, ngraham, aacid, ahmedbilal, okular-devel, kezik

Tags: Okular

Differential Revision: https://phabricator.kde.org/D20437