[electrum] don't use magic number for column identification in stake widget

Summary: If we ever add another column before txid or vout we could break the fix introduced in D20197. Let's make sure we need to change the column number in a single place to reduce the risks.

Test Plan: try sorting the stakes in the proof editor, remove stakes, add more

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D20199