BUG: Fix crash in transforms on malformed CUSTOM events.

Fixes a crash in various transforms when providing CUSTOM events whose fields
don't match the fields required for the transform.

This is fixed by requiring all `EventWindow` subclasses to supply a `fields`
property, which returns a list of strings that are required keys for any event
that can be processed by the window.  Any CUSTOM events the don't supply the
required fields for a transform window are ignored by that window.