Adds option to not load the wallet after migration

[polespinasa]

Following bitcoin/bitcoin#35266 this PR adds the option to not load the wallet after migrating to the GUI.

It is only added for the migrate option, not for the restore_and_migrate. I guess if we are restoring the wallet we always want to load it.
In any case, it's pretty straightforward to implement it there too.

Inside the original migration pop-up box it appears a checkbox that allows the user to choose if want to load the wallet or not, it is checked by default:

If yes, the wallet is loaded and shown, if not the wallet gets migrated and the GUI returns to it's state.

The checkbox has a tooltip that informs when not loading a migrated wallet can be useful:

[DrahtBot]

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Reviews

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Type	Reviewers
ACK	pablomartin4btc, hebasto, achow101
Concept ACK	w0xlt

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #943 (qt: warn about rescan in wallet migration dialogs by ShauryaaSharma)
• #bitcoin/bitcoin/25722 (refactor: Use util::Result class for wallet loading by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[pablomartin4btc]

Concept ACK

It'd be nice to have the screenshots in the description (perhaps collapsed with markup <details>) so reviewers that don't want to test it can also give their feedback.

Left a couple of nits.

[DrahtBot]

🚧 At least one of the CI tasks failed.
_{Task riscv32 bare metal, static libbitcoin_consensus: https://github.com/bitcoin-core/gui/actions/runs/30286650882/job/90045998601
LLM reason (✨ experimental): CI failed because ./ci/test/01_base_install.sh couldn’t clone the newlib-cygwin submodule from sourceware.org due to HTTP 429 (rate limited).}

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[achow101]

Concept ACK

In terms of UX, I think it would be better to have this be a checkbox option in the initial dialog rather than a second dialog that the user has to click through.

[polespinasa]

In terms of UX, I think it would be better to have this be a checkbox option in the initial dialog rather than a second dialog that the user has to click through.

Done, modified the description to show the current UI.

[pablomartin4btc]

ACK 2b4f57c

[w0xlt]

Concept ACK

[pablomartin4btc]

ACK 4cea595

[hebasto]

ACK 4cea595, I have reviewed the code and it looks OK.

[achow101]

ACK 4cea595