DimensionStructure tree CRUD

[Andras-Csanyi]

Summary
It is about managing the DimensionStructure tree which consists of DimensionStructureNodes. The possible operations are listed below.
DimensionStructureNode doesn't have a grid where they are listed. They are available only via tree management user interfaces.

These functions are published via SourceFormat api under the following names:

• SourceFormat/Tree/AddNodeAsync
• SourceFormat/Tree/DeleteNodeAsync

Definition of Done:

• Add node to tree and tests
• Delete node from tree and tests