[KPhotoAlbum] Some more things

[Robert Krawitz]

I've updated contrib/kpa-merge to handle version 8 files; it always
writes version 7. I also updated database-layout.md to note that
stack IDs and order start with 1, rather than the perhaps expected 0.
I pushed both of these to master.

Finally, I've done some improvements to startup that improve startup
time by about 15% on my system (270K files, version 8 index.xml in
compressed format). I don't expect them to be problematic, but if
someone wants to review the changes, they're on startup-performance.
The main improvement is that it only creates new path category members
if the directory path isn't already present (it was doing quite a bit
of work). I also changed the MD5 representation to be a pair of
64-bit unsigneds rather than 4 32-bit unsigneds.

Beyond that, there's not a lot of obvious fat to trim; it's either
optimize a lot of small things even more or make much bigger changes.
About 20% of startup time is spent in the Qt XML parser; much of the
remainder is spent creating the ImageInfo. To do much better we would
need a more efficient representation (perhaps a binary stream
representation), or defer as much as possible until after the GUI is
up.