Use an union to avoid the UI variable footprints for the swap flow and vice versa

[codecov-commenter]

Codecov Report

Base: 77.72% // Head: 77.72% // No change to project coverage :thumbsup:

Coverage data is based on head (2f78bb7) compared to base (0b03782). Patch coverage: 97.01% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #53 +/- ## ======================================== Coverage 77.72% 77.72% ======================================== Files 12 12 Lines 2352 2352 ======================================== Hits 1828 1828 Misses 524 524 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `77.72% <97.01%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LedgerHQ#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/LedgerHQ/app-stellar/pull/53?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LedgerHQ) | Coverage Δ | | |---|---|---| | [src/transaction/transaction\_formatter.c](https://codecov.io/gh/LedgerHQ/app-stellar/pull/53?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LedgerHQ#diff-c3JjL3RyYW5zYWN0aW9uL3RyYW5zYWN0aW9uX2Zvcm1hdHRlci5j) | `94.81% <97.01%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LedgerHQ). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LedgerHQ)

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