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/qt/forms/createwalletdialog.ui

@@ -109,6 +109,16 @@
         </property>
        </widget>
       </item>
+      <item>
+       <widget class="QCheckBox" name="external_signer_checkbox">
+        <property name="toolTip">
+         <string>Use an external signing device such as a hardware wallet. Configure the external signer script in wallet preferences first.</string>
+        </property>
+        <property name="text">
+         <string>External signer</string>
+        </property>
+       </widget>
+      </item>
      </layout>
     </widget>
    </item>
@@ -143,6 +153,7 @@
   <tabstop>disable_privkeys_checkbox</tabstop>
   <tabstop>blank_wallet_checkbox</tabstop>
   <tabstop>descriptor_checkbox</tabstop>
+  <tabstop>external_signer_checkbox</tabstop>
  </tabstops>
  <resources/>
  <connections>

src/qt/forms/optionsdialog.ui

@@ -229,6 +229,36 @@
          </layout>
         </widget>
        </item>
+       <item>
+        <widget class="QGroupBox" name="groupBoxHww">
+         <property name="title">
+          <string>External Signer (e.g. hardware wallet)</string>
+         </property>
+         <layout class="QVBoxLayout" name="verticalLayoutHww">
+           <item>
+            <layout class="QHBoxLayout" name="horizontalLayoutHww">
+             <item>
+              <widget class="QLabel" name="externalSignerPathLabel">
+               <property name="text">
+                <string>&amp;External signer script path</string>
+               </property>
+               <property name="buddy">
+                <cstring>externalSignerPath</cstring>
+               </property>
+              </widget>
+             </item>
+             <item>
+              <widget class="QLineEdit" name="externalSignerPath">
+                <property name="toolTip">
+                  <string>Full path to a Bitcoin Core compatible script (e.g. C:\Downloads\hwi.exe or /Users/you/Downloads/hwi.py). Beware: malware can steal your coins!</string>
+                </property>
+              </widget>
+             </item>
+            </layout>
+           </item>
+         </layout>
+        </widget>
+       </item>
        <item>
         <spacer name="verticalSpacer_Wallet">
          <property name="orientation">

src/qt/forms/receiverequestdialog.ui

@@ -254,6 +254,19 @@
        </property>
       </widget>
      </item>
+     <item>
+      <widget class="QPushButton" name="btnVerify">
+       <property name="text">
+        <string>&amp;Verify</string>
+       </property>
+       <property name="toolTip">
+        <string>Verify this address on e.g. a hardware wallet screen</string>
+       </property>
+       <property name="autoDefault">
+        <bool>false</bool>
+       </property>
+      </widget>
+     </item>
      <item>
       <widget class="QPushButton" name="btnSaveAs">
        <property name="text">