Refactor preview so that one component can provide multiple speech sources

[Tefached95]

Any component that is considered a speech source currently provides only one speech part component. This worked under the presumption mainly that any single workflow component will only ever provide one particular speech source. For example, a single intent reader provides only the utterances from the intent it reads.

However, custom elements, such as blocks and processors, may define several internal intent readers which are all grouped under the same element. Instead of having several intent reader elements which can each provide one speech part, this is a single element that should be able to provide multiple speech parts.

[Tefached95]

Rejected. Was not necessary.