auto levels in krita

[Alex Demko]

I added a small "auto levels" feature to krita, much like the feature
in the gimp. I noticed gimp by default presents the R/G/B histograms,
but by default krita works on the L one. I'm not a color guy, so I'm
not sure if this makes a big difference. It seems to work nicely,
though.

If the team is interested, the patches (1 for the feature, 1 to
QScopePointer a loose pointer) are here:

https://demko.ca/~ademko/kritalevels/

Feed back is appreciated. I tried to match style as much as possible.
If you 'd prefer I upload my git branch to github (or somewhere else),
I can do that.

Thanks