Optionally use images' embedded thumbnails for greater performance, even if they're too small

Summary: Only generate high resolution thumbs if the user cares to not throw them away when closing gwenview, that is when deleteThumbnailCacheOnExit() is set to false. If set to true, prefer speed over quality: always use the embedded thumbnail regardless of its size.

Reviewers: #gwenview, ngraham

Reviewed By: #gwenview, ngraham

Subscribers: yurchor, kde-doc-english, ngraham, #gwenview

Tags: #gwenview, #documentation

Differential Revision: https://phabricator.kde.org/D21330