The first part of selection tool outline optimization is done

There are at least two significant changes happened:

1) Every selection now has outlineCache() method which is supposed to
be updated asynchronously and can be in either valid or invalid state.

2) The initial outline for the selection in most of the cases should not
be calculated, because it i sknown by the tool itself.

3) The process of the "validness" controlling of the cache is done by
a transaction. If you create a non-selection transaction on a
selection paint device, the cache is cleared. If you are running a
selection transaction, you should care for the cache yourself. The
operations inside KisPixelSelection keep the cache consistent. If you
use any other operation, you should invalidate the cache
manually. This is, of course, not the ideal approach but, I guess,
something can be done to that in the future.

TODO: the actual asynchronous calculation of the selection outline

CCMAIL:kimageshop@kde.org
CCBUG:319535