Speed up construction of the history page

Review Request #111053 - Created June 17, 2013 and submitted - Latest diff uploaded

Jan Kundrát
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

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.
The speed still sucks, but at least it finishes in a finite amount of time now.

Code is available in kde:clones/rekonq/jkt/jkt-rekonq , branch speedup-history, commit 7c257d7dc1d642cfe39cb687ae7d170ef7624b44.