Bump max thumbnail size to 512px
Summary: 256 really isn't all that big on modern hidpi/4k displays.
Reviewers: ltoscano, gateau, Gwenview, rkflx, ngraham
Reviewed By: gateau, Gwenview, ngraham
Subscribers: atravneff, rkflx, ngraham
Tags: Gwenview
Differential Revision: https://phabricator.kde.org/D6083