Properly create KPixmapSequence

Summary:
The KPixmapSequence constructor needs the full path of the icon, so the
current code doesn't work and generates a "Invalid pixmap specified"
warning at runtime. By using KIconLoader we can fix this issue.

Test Plan: Search something and reach the end of document to make the animation start.

Subscribers: Okular

Tags: Okular

Differential Revision: https://phabricator.kde.org/D7996