Flash of generic thumbnail with `Create thumbnails for whole folder` unchecked

[rotbus19]

1. with `Create thumbnails for whole folder` checked, thumbnails are displayed instantly

2. with `Create thumbnails for whole folder` unchecked, there is a flash of generic image thumbnails before the actual thumbnail is displayed. This happens regardless of image type or number of images in a folder.

Affects 1.8.8 and 1.9.x

[xnview]

yes, icon of files is shown before thumbnail is extracted

[rotbus19]

but it only happens with `Create thumbnails for whole folder` unchecked - so performance / behaviour is degraded as a result of that setting