reposition_legend: Extract legend from gtable

[stefanedwards]

If legend is not given and the plot object is not ggplot, reposition_legend fails.

We should at least attempt to extract the guidebox from the gtable object, remove the guidebox grob from the gtable object.

Problem: Replacing the guidebox grob with e.g. a zeroGrob will leave an empty space - should we... ? Set the width/height of that cell to 0 (depending on it being left/right or top/bottom)? Add another bloody argument to control that behaviour?