Plotting stuff needs to move elesewhere

[j-fu]

Plotting (with PyPlot and now Makie) was added here for convenience. Dependencies are weak, as the Plotter module is passed to the plotting methods, so not much overhead when this is ignored. With the extension mechanism, finally there is a way to check for version consistency. As wee see in bdfb889, this now requires to track updates of plotting packages which is completely out of scope of this package.

So this needs to be cleaned up.

The plan:

• Copy the plotting stuff elsewhere. I consider GridVisualize.jl (which then would get a GridVisualizeTriangulateExt extension) the right place as IMHO a TriangulatePlotting package would be over the top.
• Deprecate plotting methods in Triangulate.jl (perhaps in v2.3)
• After a grace period (a couple of weeks), remove the plotting methods from Triangulate.jl. As this would be breaking, this will be v3.0.