PlotSpec interface, and changing types in convert_arguments

[asinghvi17]

Consider the following MWE:

@recipe(TestRecipe) do scene
    Attributes(;
    )
end

function Makie.convert_arguments(::Type{<: TestRecipe}, x::Vector{Point2f})
    return PlotSpec{Scatter}(x)
end

function Makie.convert_arguments(::Type{<: TestRecipe}, x::Vector{Point3f})
    return PlotSpec{Lines}(x)
end

sc = Scene(; camera = cam2d!) # we don't need to see the plot, just inspect it

testplot = testrecipe!(scene, rand(Point2f, 10))

scene.plots # 1-element array with a scatter plot

# IMO this should change the plot to lines
# but the plot stays as scatter right now.

testplot[1][] = rand(Point3f, 10)

scene.plots # 1-element array with a scatter plot, again

Right now, returning a PlotSpec from convert_arguments simply converts the plot directly, i.e., only the PlotSpec is plotted.

I would propose that the plot should be encapsulated in the original plot type, thus allowing us to change the plotspecs dynamically. That would allow text layouts to simply be argument conversions, and the linesegment etc treatment would not have to be built into the text plot! method, but rather returned as PlotSpec from convert_arguments with defined used_attributes. Seems like a pretty flexible method to me.

If the order and type of the plotspecs are the same then they should not be replotted, if not then remove all old plots by empty!(p.plots) and replot them.

Essentially, PlotSpecs should be extremely flexible types which allow the user to replot at will. This would allow flexible recipes like DifferentialEquations.jl plots to be implemented easily and efficiently, as well as allowing for different methods of rendering text (latex as images, markdown, pango, etc). It would also allow us to consume Plots.jl recipes a lot more easily.