Add IsLegacy to CWallet so that the GUI knows whether to show watchonly

Summary: Backport of Core PR16528 [35/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/ce24a944940019185efebcc5d85eac458ed26016

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8458