attempts at drag and dropping columns:
 - ulitmately failed but learned a lot
 - the rGtkDataFrame does not implement the interfaces for dragging rows, that's why it doesnt work! (could try to use a peroper TreeStore instead...)
 - could implement it with the drag and drop functions I've put in (but commented out) except GTK complains that i'm adding rather than overwriting the default drag and drop handlers (implemented in C) - not sure how to best get around this
work in progress on removing rows