Merge bitcoin-core/gui#4: UI external signer support (e.g. hardware wallet)

1c4b456e1a gui: send using external signer (Sjors Provoost)
24815c6309 gui: wallet creation detects external signer (Sjors Provoost)
3f845ea299 node: add externalSigners to interface (Sjors Provoost)
62ac119f91 gui: display address on external signer (Sjors Provoost)
450cb40a34 wallet: add displayAddress to interface (Sjors Provoost)
eef8d64529 gui: create wallet with external signer (Sjors Provoost)
6cdbc83e93 gui: add external signer path to options dialog (Sjors Provoost)

Pull request description:

  Big picture overview in [this gist](https://gist.github.com/Sjors/29d06728c685e6182828c1ce9b74483d).

  This PR adds GUI support for external signers, based on the since merged bitcoin/bitcoin#16546 (RPC).

  The UX isn't amazing - especially the blocking calls - but it works.

  First we adds a GUI setting for the signer script (e.g. path to HWI):

  <img width="625" alt="Schermafbeelding 2019-08-05 om 19 32 59" src="https://user-images.githubusercontent.com/10217/62483415-e1ff1680-b7b7-11e9-97ca-8d2ce54ca1cb.png">

  Then we add an external signer checkbox to the wallet creation dialog:

  <img width="374" alt="Schermafbeelding 2019-11-07 om 19 17 23" src="https://user-images.githubusercontent.com/10217/68416387-b57ee000-0194-11ea-9730-127d60273008.png">

  It's checked by default if HWI detects a device. It also grabs the name. It then creates a fresh wallet and imports the keys.

  You can verify an address on the device (blocking...):
  <img width="673" alt="Schermafbeelding 2019-08-05 om 19 29 22" src="https://user-images.githubusercontent.com/10217/62483560-43bf8080-b7b8-11e9-9902-8a036116dc4b.png">

  Sending, including coin selection, Just Works(tm) as long the device is present.

  ~External signer support is enabled by default when the GUI is configured and Boost::Process is present.~

  External signer support remains disabled by default, see https://github.com/bitcoin/bitcoin/pull/21935.

ACKs for top commit:
  achow101:
    Code Review ACK 1c4b456e1a
  hebasto:
    ACK 1c4b456e1a, tested on Linux Mint 20.1 (Qt 5.12.8) with HWW `2.0.2-rc.1`.
  promag:
    Tested ACK 1c4b456e1a but rebased with e033ca1379, with HWI 2.0.2, with Nano S and Nano X.
  meshcollider:
    re-code-review ACK 1c4b456e1a

Tree-SHA512: 3503113c5c69d40adb6ce364d8e7cae23ce82d032a00474ba9aeb6202eb70f496ef4a6bf2e623e5171e524ad31ade7941a4e0e89539c64518aaec74f4562d86b

src/interfaces/node.h

@@ -6,6 +6,7 @@
 #define BITCOIN_INTERFACES_NODE_H
 
 #include <amount.h>     // For CAmount
+#include <external_signer.h>
 #include <net.h>        // For NodeId
 #include <net_types.h>  // For banmap_t
 #include <netaddress.h> // For Network
@@ -110,6 +111,11 @@ public:
     //! Disconnect node by id.
     virtual bool disconnectById(NodeId id) = 0;
 
+#ifdef ENABLE_EXTERNAL_SIGNER
+    //! List external signers
+    virtual std::vector<ExternalSigner> externalSigners() = 0;
+#endif
+
     //! Get total bytes recv.
     virtual int64_t getTotalBytesRecv() = 0;
 

src/interfaces/wallet.h

@@ -118,6 +118,9 @@ public:
     //! Save or remove receive request.
     virtual bool setAddressReceiveRequest(const CTxDestination& dest, const std::string& id, const std::string& value) = 0;
 
+    //! Display address on external signer
+    virtual bool displayAddress(const CTxDestination& dest) = 0;
+
     //! Lock coin.
     virtual void lockCoin(const COutPoint& output) = 0;
 
@@ -252,6 +255,9 @@ public:
     // Return whether private keys enabled.
     virtual bool privateKeysDisabled() = 0;
 
+    // Return whether wallet uses an external signer.
+    virtual bool hasExternalSigner() = 0;
+
     // Get default address type.
     virtual OutputType getDefaultAddressType() = 0;