Add-on ordering

**Is your feature request related to a problem? Please describe.**
In some situations it might be useful to define order to execute the addons. Typical use case might be automatic translation addons, but it might be useful elsewhere.

**Describe the solution you'd like**
UI and backend to define ordering of the addons.

**Describe alternatives you've considered**
For machine translation services, there is hardcoded ordering existing, so for this particular use case the solution exists - install one addon with all desired backends and they will be executed in order.