Proposal for global selections

[Dmitry Kazakov]

We have a bug with out global selections system.
https://bugs.kde.org/show_bug.cgi?id=273260

The point is, we store global selection as a KisSelection inside a KisImage
object. Most of our actions like Crop/Resize/Scale the image do not take
this selection into account. That is why after most of them selection
becomes shifted and non-transformed. And we can't apply a visitor to it,
because it is not a node.

I propose a solution for this problem:
KisImage will store a KisSelectionMask inside and will put this mask as a
child to the root node. This assignment will be done in two functions
setGlobalSelection() and setRootNode().

After this change, all our node visitors will work as usual, they will just
process all the children of the root node, so the global selection will be
transformed as a regular selection mask.
No change will be needed for the visitors (except for some of them, those do
not work with masks at all).

UI problem:
If the mask is added to the node graph, then it is shown in a KisLayerBox.
So we will have to add an option to KisConfig to hide it as we do for the
root layer. Actually, we may leave a way for a user to show it. It worked
quite well, when i tried to add it in a hardcoded way. The only trouble i
got was that it was impossible to toggle "Active" property of the mask.
Everything rest was working very well.

Why we can't do it in another way:
Because all the visitors and most of the commands in Krita work with nodes.
So, for example, to shift a global selection (setX(), setY() methods) we
will have to create one more command. We will have to modify/duplicate all
our visitors/commands to let them work with selections.

How it is done in other editors:
I tried Gimp. It transforms global selection with the image. And PS7, as far
as i remember, crops the selection alongside with the image as well.

-- 
Dmitry Kazakov
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