Add WindowEncoder, which support various activations

[danieldk]

Description

Window encoder that supports multiple element-wise activations.

Currently very drafty, mostly for some experiments with @kadarakos.

Types of change

Feature

Checklist

• [ ] I confirm that I have the right to submit this contribution under the project's MIT license.
• [ ] I ran the tests, and all new and existing tests passed.
• [ ] My changes don't require a change to the documentation, or if they do, I've added all required information.