Rework of iterators

[Cyrille Berger]

> Krita is at an important crossroads - lets please go the right way - as we
> have already decided.

I might not have fully understand, what do you call a pixel iterrator ? At 
line 49 of kis_pixel_iterator.h you return a Q_INT8 which is a QUANTUM !
In mymind, a pixel iterrator is something that return a structure representing 
a pixel like in KisPixelRepresentation (on which we have agreed).

-- 
--- Cyrille Berger ---