Legends for continuously styled variables

[greimel]

Here is some refactoring of the legend creation to simplify adding legends for continuously styled variables.

It also includes a proof of concept for markersize. It builds on the Colorbar PR.

I guess the goal here is to find a general solution for continuous styles (linewidth, alpha, what else?)

addresses #83 and (partially) #57

using RDatasets: dataset
using AlgebraOfGraphics, AbstractPlotting, GLMakie
using AlgebraOfGraphics.MakieLayout

mpg = dataset("ggplot2", "mpg")
cols = style(:Displ, :Hwy)
grp = style(color = :Cyl => categorical)
mpg[!,:Group] = mpg.Cyl .> 6
mpg[!,:Color] = 3 .* rand.() .+ 2 .* mpg.Cyl 

aog2 = data(mpg) * style(:Displ, :Hwy, layout_x = :Drv, markersize = :Color,  marker = :Group => categorical, color = :Color) * spec(Scatter)

scene, layout = layoutscene()
 scat = AlgebraOfGraphics.layoutplot!(scene, layout, aog2)

[greimel]

Implemented in piever/SplitApplyPlot.jl#32