Speed up construction of the history page

The old version was taking ages to even appear on my setup ("ages" being defined
as many minutes at least). Callgrind was crashing, perhaps due to webkit's JIT,
so I simply sampled the backtrace "randomly" by hand to see where the most CPU
time was being spent. These two methods came up way too often, so I did the
following:

1) Do not use KIconLoader within the inner loop, it's slow. It was slow when I
debugged too long startup of KPhotoAlbum, it produced visible file IO, and it is
trivial to speed this up. However, it was still taking time.

2) Do not request the ".historyitem" over and over again. A cached "pre-found"
copy (we're still cloning it) seems to help.

Together, these make it possible to load the history in <4 minutes here. It's
still a completely insane amount of time, but at least it appears, ultimately.

REVIEW:111053