Introduce CompressibleObjectTree

[joaomoreno]

Details:

• To facilitate adoption, it should be as similar to ObjectTree as possible with the following differences:
  • It demands renderers which know how to render compressed elements
  • It has an utility method to return the compression group an element might be in
• It should have a enableCompression property which can be toggled at runtime
  • Note that focus/selection might change when this property is toggled
• It is sightly more expensive than using the ObjectTree directly, since every element, even if uncompressed, is wrapped by a ICompressedTreeNode, so remember to recommend using the ObjectTree if compression isn't required