Makie 0.17 support

[lorenzoh]

When using Makie.jl 0.17, there are some errors cropping (see https://github.com/FluxML/FastAI.jl/runs/6404954707?check_suite_focus=true) up in the code for showing multiple observations, here:

FastAI.jl/src/interpretation/makie/showmakie.jl

Lines 54 to 58 in 2f227aa

	# Add titles to named blocks
	M.Label(grid[0, 1], "", tellwidth = false, textsize = 25)
	for (i, title) in enumerate(header)
	M.Label(grid[1, i], title, tellwidth = false, textsize = 25)
	end

This code tries to add titles to the beginning of each column of a previously created GridLayout.

[lorenzoh]

Current plan is to create a separate package FastMakie.jl that holds the Makie-related code. This way, we won't need Requires.jl and FastAI.jl + domain libraries can depend on MakieCore.jl to define recipes for visualizing their Blocks