Default to cashaddr in most of the Qt API

Summary:
As per title.

Depends on D3536, D3543 and D3549

Test Plan: Run the bitcoin-qt client with usecashaddr=0 and check that most addresses are cashaddr.

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3551