Propose shield fixes

[pacu]

Changes

• use zcash_protocol's networkType and remove unused variable
• change use of deprecated BalanceError in favor of zcash_protocol one
• Use Zip32 crate instead of re-pub of librustzcash (same version)
• remove usage of deprecated ShieldedProtocol in favor of zcash_protocol one
• remove NonNegativeAmount use in favor of Zatoshis
• Use shielding threshold as Zashi and Mobile SDKs do

Tests

No tests altered.

Issues

#106
Closes #106

[CLAassistant]

All committers have signed the CLA.

[irubido]

added UI, button disables below 0.001 ZEC