add default request expiry to preferences/config

electrum/gui/qml/components/controls/RequestExpiryComboBox.qml

@@ -0,0 +1,36 @@
+import QtQuick 2.6
+import QtQuick.Controls 2.3
+
+import org.electrum 1.0
+
+ElComboBox {
+    id: expires
+
+    textRole: 'text'
+    valueRole: 'value'
+
+    model: ListModel {
+        id: expiresmodel
+        Component.onCompleted: {
+            // we need to fill the model like this, as ListElement can't evaluate script
+            expiresmodel.append({'text': qsTr('10 minutes'), 'value': 10*60})
+            expiresmodel.append({'text': qsTr('1 hour'), 'value': 60*60})
+            expiresmodel.append({'text': qsTr('1 day'), 'value': 24*60*60})
+            expiresmodel.append({'text': qsTr('1 week'), 'value': 7*24*60*60})
+            expiresmodel.append({'text': qsTr('1 month'), 'value': 31*24*60*60})
+            expiresmodel.append({'text': qsTr('Never'), 'value': 0})
+            expires.currentIndex = 0
+            for (let i=0; i < expiresmodel.count; i++) {
+                if (expiresmodel.get(i).value == Config.requestExpiry) {
+                    expires.currentIndex = i
+                    break
+                }
+            }
+        }
+    }
+
+    onCurrentValueChanged: {
+        if (activeFocus)
+            Config.requestExpiry = currentValue
+    }
+}