Set non-overlapping legend position by default

[lmiq]

Having the default legend position to assume a minimum-overlapping position with the data would be nice. Matplotlib already does that, and apparently (see https://github.com/JuliaPlots/Plots.jl/issues/3399#issuecomment-809493284) Plots already passes the :best symbol to the backends. So if GR had that option, this could be the default behavior for GR as well.

I would be willing to work on this if I'm give any clue on where is that defined.

A tenative code for doing that (which may be put in Plots, actually), is here: https://discourse.julialang.org/t/making-plots-place-its-legend-in-a-better-spot/74986/11?u=leandromartinez98