Use QImageReader's auto transform feature for JPEG images

Summary:
QImageReader can rotate JPEG images according to their
orientation since Qt 5.5.

Reviewers: #gwenview, muhlenpfordt

Reviewed By: #gwenview, muhlenpfordt

Subscribers: muhlenpfordt

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D15238