[KPhotoAlbum] Rebuilding thumbnails is not an exercise for the faint of heart

[Tobias Leupold]

> Certainly there should be a warning issued when someone
> requests rebuilding all of the thumbnails; accidentally doing that
> might be very time consuming.  It doesn't render the program useless
> while it's going on, but it is certainly a big inconvenience.

You're totally right! I thought we had a warning message like "This could take 
a lot of time"? I'll check that.

For quite some time, KPA can generate the thumbnails on request, if it's 
needed and not there, it's generated. We (mostly Johannes ;-) implemented this 
back then for this very reason that rebuilding the whole cache is a big task.

Tobias