Replace plottertypes by Singleton

[jpthiele]

As proposed in #108 this changes dispatch.jl to use Singletons instead of using the set of nested ifs in plottertype with ismakie() and similar.

Since MakieType was heuristic based I had to introduce the alternative AbstractMakieType and change the Makie extension accordingly by adding where {MakieType <: AbstractMakieType} where needed.
Then all tests ran locally without problem.

Now, extending GridVisualize with another plotting backend can be done without needing to touch the main package.
Nevertheless, introducing a new shorthand through const AnotherPlotterPackageType = PlotterType{:AnotherPlotterPackage}
is certainly nice and quickly done.

@j-fu do you know of any downstream package that might explicitly use MakieType?

And if you are fine with this we should test the alternative to the example to remove the !private! functions ismakie and similar.

[jpthiele]

Had a quick search on GitHub and all that came up were forks of GridVisualize or completely unrelated projects when it comes to MakieType.

[j-fu]

I think formally this would be a breaking change which by semver increases the major version number and we very much should try to stick to this. I would put this on a list with other possibly breaking changes, which together would be substantial enough to be worth a najor release.