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

Fixed an NPE in the drag-and-drop handling when dropping onto an empty report. This caused trouble as empty reports have no layouted bands to lookup. 

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.