undo of selection

[Casper Boemann]

Hi again

The way selections are now created and removed on the fly gives us problems 
with undo.

When a selection is removed the undo info is also obsoleted.

In my mind we need to have a single KisSelection for the lifetime of the 
owning paintdevice (though created the first time it's referenced). Clear 
should modify the selection not delete it.

Alternatively, all transactions that delete a selection should not actually 
delete it, but  only remove it and store it in a subclass of KisTransaction)
-- 
best regards / venlig hilsen
Casper Boemann