Simplify input field validation for addresses

Summary: This logic is less restrictive and allow to input cashaddr without prefix.

Test Plan:
Compile bitcoin-qt and input a cashaddr without prefix in the amount field when sending coins.
Added unittests.

Reviewers: schancel, dagurval, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D838