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