This renames the struct `Sequence` to `Strip`.
While the motivation for this partially comes from
the "Sequence Design" #131329, it seems like this
is a good refactor whether the design gets implemented
or not.
The `Sequence` represents what users see as strips in the
VSE. Many places in the code already refere to a `Sequence`
as "strip". It's the C-style "base class" of all strip types.
This also renames the python RNA type `bpy.types.Sequence`
to `bpy.types.Strip` which means that this technically breaks
the python API.
Pull Request: https://projects.blender.org/blender/blender/pulls/132179
