Refactoring of matplotlib legend code

[philippjfr]

Currently the way legends are handled is a bit awkward as there are two possible places the legends can be drawn from:

1. The label of each Element
2. The multi-dimensional index in NdOverlay

These are currently handled semi-separately, the Element labels are set on the artists directly, while the OverlayPlot looks through the handles of all the subplots to associate the NdOverlay index with a particularly artist.

Instead each ElementPlot should simply be passed this index and set the legend options directly in the plotting call. This probably fits into a more general system, which would make the NdLayout, GridSpace, NdOverlay dimensions and corresponding values available to each ElementPlot.

[philippjfr]

This was actually done a long time ago.