#664 Finer control of plot element position (layering)

[Ivar Christopher]

In the current Layout tab, it is possible to select a plot element and "Move plot element above others". It would be useful, especially on plots with many elements, to have available the more atomic actions of "move plot element up one" and "move plot element down one".

If that functionality is currently available elsewhere, it would be convenient/intuitive to have it available through the right-click interface on the Layout tab.