Blender Git Loki

Alembic: add support for archive layers

This adds support for Alembic archive layers for the Blender CacheFile
and the Cycles Alembic procedural.

Layers are a feature of the Alembic library in which an archive can be
opened from a list of archives or file paths, and where archives
override data from the others. The order of the files matter since
latter files override data from former ones (i.e. a stack).

This feature is useful to, for example, replace a UV map or the
animation of an object without having to re-export the entire scene;
only the affected hierarchy needs to be re-exported, and "imported" as a
layer.

This adds a UI list for adding layers to the CacheFile, with buttons to
change their relative order. The filepath of the CacheFile is
implicitely considered as the bottom layer, although it is not shown as
such in the UI.

In the future, maybe with T68933, this could be the basis for a non-
destructive workflow where edits to an Alembic archive are exported as
layers.