basis.dragdrop: various improvements on MoveableElement class, DeltaWriter class implemented

- dragging limits based on minDeltaX/Y and maxDeltaX/Y that may be
calculated before drag start
- emit_start always emit with zero deltaX/deltaY
- move fixLeft, fixRight, fixTop, fixBottom from DragDropElement to
MoveableElement
- remove DragDropElement#containerGetter
- MoveableElement is more flexible now (able to change any properties
not only top/left)
- DeltaWriter and related classed implemented, helpers that could
resolve what property to change and write changes into element