Page changes triggered by mouse clicks should all happen on button release

Summary: Previously this was inconsistent: left button clicks triggered page changes at release, but right button and forward/backward button clicks triggered directly at the mousePressEvent.

Reviewers: Okular, ngraham, aacid

Reviewed By: ngraham

Subscribers: aacid, okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D19702