iterators and selection

[Casper Boemann]

Just to make sure nobody missed it

The iterators now have an isSelected() method. This is much faster to check
against than to have to create a KisPixel, set the selectedness, and then
checking that.

Should any of you feel the need to extend the trait to also set the
selectedness of returned pixels then be my guest, but for fast checking,
please use the isSelected() method.

Oh and the invert filter uses this new feature (one line of code - now that
was easy)

best regards / venlig hilsen
Casper Boemann