Implement KisForest container for tree-like structures

Historically, we implemented tree-like structures by putting
the linked-lists inside the objects themselves (see KisNode and
KoShape). It it not very convenient in cases when you need a custom
hierarchy over existing objects, e.g. for composing.

KisForest implements a container for building tree-like structures from
arbitrary objects. The stored elements don't have to have know anything
about hierarchy and, more than that, they may be contained in multiple
different hierarchies.

See KisForestTest for examples on how to use the forest.

Ref T11969
CCBUG:392242
CC:kimageshop@kde.org