Allow bundling speech dictionaries in add-ons

[LeonarddeR]

Is your feature request related to a problem? Please describe.

It is currently possible to bundle symbol dictionaries in add-ons. It is however not possible to bundle speech dictionaries.

Describe the solution you'd like

1. Add the ability to bundle speech dictionaries
2. Reconsider speech dictionaries gui. Add the ability to override add-on dictionaries on the default, voice or temporary level.

Describe alternatives you've considered

None

Additional context

Bundling speech dictionaries in add-ons would be an interesting feature for add-ons that facilitate learning foreign languages. For example, when providing pronunciation rules for greek words like λογος, we can make speech pronounce λ like l, ο like ò as in hot, etc.