PRD-4445: Fixed an NPE in the drag-and-drop handling when dropping onto an empty report. Also fixed precision issues where dropping an element onto an empty root-band placed the elements on a different location than the drop position indicated by the mouse.

Code smell: Refactored drag-and-drop handling and condensed duplicate code.
Merge pull request #32 from tmorgner/master

PRD-4445 - fixed bordercase crash introduced earlier in PRD-4313