[electrum] seed completer: colour words yellow if only in old electrum list

Summary:
Some people complained that due to merging the two word lists,
it is difficult to restore from a metal backup, as they planned
to rely on the "4 letter prefixes are unique in bip39 word list" property.
So we colour words that are only in old list.

This is a backport of https://github.com/spesmilo/electrum/commit/7120c344b2d12b60be8c5fe82e7a9964425075c9
Depends on D17569

Test Plan: Restore a wallet from seed, check that some of the suggested words are now yellow. Check that these words are not part of the BIP39 english wordlist (https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17570