[wallpapers/image] Seed random number generator

Summary:
Otherwise when we randomise the wallpaper order we will always get the
same results.

CCBUG: 413463

Test Plan: qDebug + ran twice

Reviewers: Plasma, ngraham, davidre

Reviewed By: ngraham, davidre

Subscribers: davidre, apol, anthonyfieroni, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24985