[electrum] enable sorting when selecting utxos for a proof
Summary:
For wallets with many coins this can help with finding the coins to add to an avalanche proof, by sorting by amount or block height.
Depends on D18989
Test Plan: try the proof editor, add and remove coins, click the column headers to select a sorting column and toggle ascending and descending order
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D18990