Electrum plugin: Support multisig wallets via script descriptors

[shesek]

Depends on #1 (nearly ready!) and rust-bitcoin/rust-miniscript#164.

[shesek]

Implemented on the 202010-electrum-multisig branch: ff94a6c

Basically works, apart from the lack of sortedmulti() support.

[shesek]

Implemented in 52e9d3a!