Fix some warnings in CEGUI layouts.
Tree widget implementation

- ScrollTreebox and its child CEGUITreebox
  - Implement tree widget with one level hierarchy (flat groups)
- CEGUIExtendedTree and CEGUIExtendedTreeItem extend CEGUI::Tree widget
  - TreeItem is now aware of its parent in hierarchy
  - Tree selection is possible for closed branches (setItemSelectState
    to be fixed)
  - Selection count recurses inside child items
  - Introduced method to get selected leaves - items without children
- TreePopup is a dialog with tree widget that returns selected items
- Object children selection in ObjectEditor switched from ListPopup to
  TreePopup
- Implemented simple division of objects into groups or rather sets
  according to ScrollEd implementation