Fixed autoscrolling bug in Krita

I've extracted one method from KoToolProxy and overridden it in newly
created KisToolProxy. This method just converts widget coordinates
into document coordinates.

Fixed a bug reported by Thorsten.

WARNING: Be careful, the definition of 'document origin' differs in
Krita and other flake-based applications. In Krita 'origin' is used
only when the size of the image is less than the size of the image and
vast scrolling is disabled. It is used for centering and means "the
offset from the topLeft of the widget to the corner of the
document". In flake, 'origin' means "the offset of the document in a
big white rectangle that surrounds the document". Krita has no such
rectangle and the origin is used in a different way due to historical
reasons, i guess.


CC,