Disallow encryption of watchonly wallets

Watchonly wallets do not have any private keys to encrypt. It does not
make sense to encrypt such wallets, so disable the option to encrypt
them.

This avoids an assertion that can be hit when encrypting watchonly descriptor
wallets.

Github-Pull: bitcoin-core/gui#631
Rebased-From: 4c49541

src/qt/bitcoingui.cpp

@@ -1310,6 +1310,12 @@ void BitcoinGUI::setEncryptionStatus(int status)
 {
     switch(status)
     {
+    case WalletModel::NoKeys:
+        labelWalletEncryptionIcon->hide();
+        encryptWalletAction->setChecked(false);
+        changePassphraseAction->setEnabled(false);
+        encryptWalletAction->setEnabled(false);
+        break;
     case WalletModel::Unencrypted:
         labelWalletEncryptionIcon->hide();
         encryptWalletAction->setChecked(false);

src/qt/walletmodel.cpp

@@ -305,6 +305,11 @@ WalletModel::EncryptionStatus WalletModel::getEncryptionStatus() const
 {
     if(!m_wallet->isCrypted())
     {
+        // A previous bug allowed for watchonly wallets to be encrypted (encryption keys set, but nothing is actually encrypted).
+        // To avoid misrepresenting the encryption status of such wallets, we only return NoKeys for watchonly wallets that are unencrypted.
+        if (m_wallet->privateKeysDisabled()) {
+            return NoKeys;
+        }
         return Unencrypted;
     }
     else if(m_wallet->isLocked())

src/qt/walletmodel.h

@@ -71,6 +71,7 @@ class WalletModel : public QObject
 
     enum EncryptionStatus
     {
+        NoKeys,       // wallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)
         Unencrypted,  // !wallet->IsCrypted()
         Locked,       // wallet->IsCrypted() && wallet->IsLocked()
         Unlocked      // wallet->IsCrypted() && !wallet->IsLocked()