(8.6-en) Drag and drop to rebuild requirement hierarchy

1. Expand a requirement to make its child requirements (source) visible for you.
2. Expand another requirement (destination).
3. Drag source requirement to destination requirement, then drop it.
4. The source requirement will be presented as a child requirement.
   Note: all child requirements will be moved along with its parent requirement.

[Screenshot - UG83_Requirements_Panel_0004]