Make native segwit default by always adding NATIVE_SEGWIT to
defined constants in configure.fsx.

To make tests work properly with native segwit, rename
deprecated OriginAddress property to OriginMainAddress
in test data.

Remove NATIVE_SEGWIT compile-time constant and all code that
was disabled when NATIVE_SEGWIT was defined.

Also remove readonly accounts migration to native segwit.

Added option to import wallet using BIP39 seed phrase
(mnemonic). All funds from imported wallet (currently only
first receiving address is used) are sent to BTC account of
choice. After that, imported account is converted to archived
account so geewallet will warn the user if more funds arrive
to it in the future.