selection refactor

[Casper Boemann]

Hi krita devs

I'm thinking of putting one last refactor in before the freeze.

Regarding selections. At the moment we have no selection and select all, which 
is essentially the same from the user's perspective.

I would like to combine these to make it simpler for the user. I don't think 
the internal refactor is that big.

What do you think?

-- 
best regards / venlig hilsen
Casper Boemann