[KPhotoAlbum] Improving loading images when they are to be stacked

[Robert Krawitz]

I've noticed for a while that loading images when they need to be
stacked is much slower than otherwise. I thought it was related to
the datebar flashing, but that appears to only be one piece of it.
There are a number of other problems, including processing EXIF
information with every image and the process of identifying stacks
altogether.

This patch fixes the datebar flashing (don't update the datebar until
all images are loaded), the EXIF information (likewise), and part of
the problem of identifying stacks (creating a cache while searching
for images that's destroyed right away). This is somewhat faster,
from maybe 1-2 images/second to about 6, but nowhere near the speed of
loading images that don't need to be stacked, which I get about 100
images/second (for ~800KB images). So there's still a lot of
inefficiency in there, but I haven't made a lot of headway through
it.

[Robert Krawitz]

So that last patch had a turd in it...this is cleaned up.