gui: show watch-only balance in send screen

Summary:

• [test] qt: add send screen balance test
• [gui] send: show watch-only balance in send screen

This is a backport of Core PR17587

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8076