Several bug fixes of AE to Spine export script.

- Pre-calculate opacity of parented and/or pre-composed layers.
- Export linear and hold interpolation type.
- Sample properties at constant frame rate if bezier interpolation used.
- Determine unique footage/layer name on-the-fly without changing composition.
- Insert intermediate rotation keys if required.
- Refactored script to make decomposition optional. The setup pose is not correctly computed yet, however, thus decomposition still the required default.
Fix bug in sparsifyKeyframes.
Fix slot color animation. Parent opacity has no influence on layer.