Add support for Inner>Outer>Inner perimeter order

[Epsylon42]

This pull requests replaces the external_perimeters_first boolean option with a perimeters_order enum and adds support for Inner>Outer>Inner perimeter order, similar to one present in some other slicers.

I also fixed an inconsistency between the Classic and Arachne perimeter generators. Previously brim would affect first layer perimeter order in the Classic but not in Arachne. Now it affects both, basically overriding this option for the first layer and always printing outside-in as a continuation of the brim. (To be honest i'm not sure if that's necessary, but I tried to preserve as much of previous behavior as possible)

• I tested basic printing with this option on my own printer and the results were good
• I did not test the changes in scarf joint code that were required in SeamPlacer.cpp. The change seems trivial-ish and should not change behavior for Inner>Outer and Outer>Inner modes. For the Inner>Outer>Inner mode I assumed that it should work the same as Outer>Inner. If someone can tell me how to test if this is correct (what to print and where to look) - I will.

Closes #9393
Closes #14888
Possibly closes #8949
And possibly some other requests for the same feature