Add HiDPI support for thumbnails

Summary:
The idea is to localize changes in ThumbnailView as much as possible:
ThumbnailView::thumbnailSize() returns the size in device independent
pixels, i.e. it seems from the outside that ThumbnailView behaves as
well as before this change. But, of course, item delegates must take
into account that ThumbnailView::thumbnailForIndex() will return
a pixmap with the devicePixelRatio set.

CCBUG: 373178

Reviewers: #gwenview, ngraham

Reviewed By: #gwenview, ngraham

Subscribers: ngraham

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D20267