Clean up colormap handling and implement colorscale + alpha kw

[SimonDanisch]

Continuation of #colorscale #2493

[tomchor]

maybe we'll also need a few more tests!

I think the more we can test this PR for corner cases the better, so @tomchor, if you have any reduced examples from your production codes that we didn't think of, they would be very welcome in this PR.

All my production code is actually pretty simple in the use of logscale. The only difference that I didn't see tested before (which is why I wanted to test my code) is that I use for recording animations with the cairo backend.

[SimonDanisch]

interactions/animations with colorbar etc is currently the most untested..

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[SimonDanisch]

I think this is ready to merge :)

[SimonDanisch]

To not make this breaking we should rather deprecate hexbin.scale instead of just removing it.