Add @auth_protect decorator.

This guards function calls by storing the function, args and kwargs into
an added attribute '__auth_fcall' on the object using the decorator,
then emits a signal that can be handled by the UI.

The UI can signal auth-success or auth-failure back to the object by
calling either the authProceed() slot or the authCancel slot.

The object utilizing this decorator MUST inherit/mixin the AuthMixin
class, which provides the above two slots, and handling of state.

The decorator also accepts a 'reject' parameter, containing the name of
a parameterless function on the object, which is called when
authentication has failed/is cancelled.

electrum/gui/qml/components/main.qml

@@ -221,4 +221,18 @@ ApplicationWindow
             notificationPopup.show(message)
         }
     }
+
+    Connections {
+        target: Daemon.currentWallet
+        function onAuthRequired() {
+            var dialog = app.messageDialog.createObject(app, {'text': 'Auth placeholder', 'yesno': true})
+            dialog.yesClicked.connect(function() {
+                Daemon.currentWallet.authProceed()
+            })
+            dialog.noClicked.connect(function() {
+                Daemon.currentWallet.authCancel()
+            })
+            dialog.open()
+        }
+    }
 }