[Qt] Add simple optin-RBF checkbox and confirmation info

src/qt/forms/sendcoinsdialog.ui

@@ -1158,6 +1158,16 @@
              </item>
             </layout>
            </item>
+           <item>
+            <widget class="QCheckBox" name="optInRBF">
+             <property name="text">
+              <string>Request Replace-By-Fee</string>
+             </property>
+             <property name="toolTip">
+              <string>Indicates that the sender may wish to replace this transaction with a new one paying higher fees (prior to being confirmed).</string>
+             </property>
+            </widget>
+           </item>
           </layout>
          </widget>
         </item>
@@ -1168,8 +1178,8 @@
           </property>
           <property name="sizeHint" stdset="0">
            <size>
-            <width>800</width>
-            <height>1</height>
+            <width>40</width>
+            <height>5</height>
            </size>
           </property>
          </spacer>

src/qt/sendcoinsdialog.cpp

@@ -245,6 +245,8 @@ void SendCoinsDialog::on_sendButton_clicked()
     else
         ctrl.nConfirmTarget = 0;
 
+    ctrl.signalRbf = ui->optInRBF->isChecked();
+
     prepareStatus = model->prepareTransaction(currentTransaction, &ctrl);
 
     // process prepareStatus and on error generate message shown to user
@@ -324,6 +326,13 @@ void SendCoinsDialog::on_sendButton_clicked()
     questionString.append(QString("<span style='font-size:10pt;font-weight:normal;'><br />(=%2)</span>")
         .arg(alternativeUnits.join(" " + tr("or") + "<br />")));
 
+    if (ui->optInRBF->isChecked())
+    {
+        questionString.append("<hr /><span>");
+        questionString.append(tr("This transaction signals replaceability (optin-RBF)."));
+        questionString.append("</span>");
+    }
+
     SendConfirmationDialog confirmationDialog(tr("Confirm send coins"),
         questionString.arg(formatted.join("<br />")), SEND_CONFIRM_DELAY, this);
     confirmationDialog.exec();