[KPhotoAlbum] Thumbnails missing with latest git

[Tobias Leupold]

> What is the purpose behind the incremental thumbnails?

I can at least answer this one: If the thumbnail size is changed, the default 
KPA behavior was that _all_ thumbnails were recalculated, which is painfully 
slow if the collection is on NFS. KPA even was e. g. unresponsive for about 15 
minutes for "only" 8000 photos in my case.

With the "on-demand" thumbnails, they are built as they are requested 
(viewed). So the lag distributes to many small operations instead of one big. 
This way, KPA keeps being responsive with only small lags, which is surely a 
more desirable behavior. For local collections the lag should be that short 
that the user won't really notice the thumbnail building. So in this case it 
doesn't hurt, and for NFS users, it's far better.

Cheers, Tobias