Exclude underscore-prefixed Matplotlib themes from selection

[QuLogic]

The preferences show all built-in Matplotlib styles:

The underscore-prefixed ones are internal, and really shouldn't be offered as an option.

[itprojects]

Reducing the number of ways to display is not necessary.
If you do not like them, do not use them.

[QuLogic]

I'm not saying I don't like them; I'm saying as a Matplotlib developer they shouldn't be exposed to be used.

[QuLogic]

But actually, I just realized you're using style.available, so really we should fix this ourselves.

[itprojects]

So if it's fixed in matplotlib then it will be automatically fixed in MasVisGtk?

[QuLogic]

Yes, we should take care of it.

[QuLogic]

This will be fixed by Matplotlib 3.11.