After Effects export: add solid and image plane export

This squashed commit also:
- sets the default AE comp name to the blend file name

- reverts "io_export_after_effects: update to 2.8 T63856"
(commit 5781362d)
I don’t know what happened there, but it actually removed some
features, most notably rBA31e5c6b9.

- does a bit of cleanup, formatting, and refactoring:
  - include main() function in operator
  - use loop for prop animation writing
  - fix long-standing bug where spot lights wouldn't export cone angle
    and blend
  - simplify selection data structure