[electrum] support parsing multi-output URIs

Summary:
This adds support for multi-outputs URIs in the send tab.
For now, the receive tab is still limited to generating regular single-output URIs.

Depends on D15385

Test Plan:
Paste the multi-output URI examples from the added test and from the bip21-ecash-additions.md document into the payto field, and check that the transactions are created as expected.

Test also other examples from the test_bip21 test suite in the GUI, especially the exotic ones such as ecash:?op_return_raw=04deadbeef (takes one input randomly and creates an op_return output and a change output) and ecash:?r=http://domain.tld/page?h%3D2a8628fc2fbe (this one will not work because the bip70 url is invalid, but it should still try to resolve it and give a meaningful error message).

python test_runner.py

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15250