Refactor navigation

[obulat]

Description

This PR does some refactoring before adding more Vue Vocabulary components. Namely, the navigation for stepper (Next and Previous buttons) were extracted into a separate component, and FirstStep and Step components were combined into a more general one, ChooserStep.

Checklist

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• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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