[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