Merge #16944: gui: create PSBT with watch-only wallet

c6dd565c88 [gui] watch-only wallet: copy PSBT to clipboard (Sjors Provoost)
39465d545d [wallet] add fillPSBT to interface (Sjors Provoost)
848f889208 [gui] send: include watch-only (Sjors Provoost)
40537f0909 [wallet] ListCoins: include watch-only for wallets without private keys (Sjors Provoost)

Pull request description:

  For wallets with `WALLET_FLAG_DISABLE_PRIVATE_KEYS` this makes the watch-only balance available on the send screen (including coin selection). Instead of sending a transaction it generates a PSBT.

  The user can take this PSBT and process it with [HWI](https://github.com/bitcoin-core/HWI) or put it an SD card for hardware wallets that support that.

  The PSBT is copied to the clipboard. This was the easiest approach; we can add a dialog later to display it, as well as an option to save to disk.

ACKs for top commit:
  instagibbs:
    test and code review ACK https://github.com/bitcoin/bitcoin/pull/16944/commits/c6dd565c8820aa8a98b190621e10c6e2821a9ecc
  meshcollider:
    re-ACK c6dd565c88

Tree-SHA512: ebc3da0737e33b255ed926191b84569aedb6097d14868662bd5dce726ce3048e86e9a31eba987b10dffe1482b35c21ae1cd595c2caa4634bc4cf78a826a83852

src/interfaces/wallet.cpp

@@ -18,8 +18,9 @@
 #include <wallet/feebumper.h>
 #include <wallet/fees.h>
 #include <wallet/ismine.h>
-#include <wallet/rpcwallet.h>
 #include <wallet/load.h>
+#include <wallet/psbtwallet.h>
+#include <wallet/rpcwallet.h>
 #include <wallet/wallet.h>
 
 #include <memory>
@@ -357,6 +358,14 @@ public:
         }
         return {};
     }
+    TransactionError fillPSBT(PartiallySignedTransaction& psbtx,
+        bool& complete,
+        int sighash_type = 1 /* SIGHASH_ALL */,
+        bool sign = true,
+        bool bip32derivs = false) override
+    {
+        return FillPSBT(m_wallet.get(), psbtx, complete, sighash_type, sign, bip32derivs);
+    }
     WalletBalances getBalances() override
     {
         const auto bal = m_wallet->GetBalance();