Updating deprecated functions

[JL VT]

When I compile, I see a huge list of warnings that certain functions
being used are deprecated (I think it's all Iterators).

Before I start on an epic crusade of updating all those function
references, what benefit should Krita see from the use of the new
functions?. More stability?, More speed?, A certain feature?.

If it helps with stability (e.g.: less crashes) then I'll try to
update them for 2.3.