add support for vertical spacing in grid layout
update the demo to show vertical spacing in grid
Merge pull request #113 from jwilling/grid-vertical-spacing

Grid vertical spacing
- Added drag-and-drop support
Merge remote-tracking branch 'dhoogenb/feature/Drag-and-drop_support'
make relevant drag&drop headers publicly visible (not just within the project)
re-order mouseUp event hierarchy so that super is called last