1) Improvement: Improve secret export
+ Private keys now export in standard WIF format instead of raw hex, so they can be imported directly into other wallets.
+ Allow users to export an extended private key (zprv) that can restore the entire wallet, including every address, in another wallet.
2) Fixed: "First receiving address" on the Send screen did nothing when used with a coin that had never received funds. It now correctly returns the first address.