Fixes #2663 - No more nested classes

[tznind]

Fixes #2663 - Promote all nested classes to full classes

In #2406 many classes were moved from nested to root level. This PR explores moving all classes (except those in SpinnerStyle.cs) out to their own classes.

In some cases e.g. Curses.Window I named the new class CursesWindow which is I think a nice way of grouping things together. See also e.g. Bar (previously BarSeries.Bar) is now BarSeriesBar

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• I've named my PR in the form of "Fixes #issue. Terse description."
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[tig]

Please back out the changes to the ConsoleDrivers and related code. I will take care of this in #2612; it's diverged enough that merging from this PR will be a PITA. Thanks.

[tznind]

Please back out the changes to the ConsoleDrivers and related code. I will take care of this in #2612; it's diverged enough that merging from this PR will be a PITA. Thanks.

I've reverted everything in the ConsoleDrivers folder to match v2_develop.

I have commented out <WarningsAsErrors>CA1034</WarningsAsErrors> (in the .csproj file) so you can work on the console classes without seeing errors about nesting. But I think it is nice to have as it prevents us creating public nested classes in future (will result in build failure) so if we could turn it back on after the console refactor that would be good.

You can still suppress in source:

#pragma warning disable CA1034 // Nested types should not be visible

[...]

#pragma warning restore CA1034 // Nested types should not be visible