Add "Done" button when verification session is finished

It isn't immediately obvious when to close the window, other clients
(e.g. Element Web) has a "Got It" button in addition to the regular
dialog close button.

src/app/qml/KeyVerificationDialog.qml

@@ -171,7 +171,8 @@ Kirigami.Page {
                     return "";
                 }
             }
-            onDone: root.QQC2.Window.window.close()
+            isDone: root.session.state === KeyVerificationSession.DONE
+            onDone: root.closeDialog()
         }
     }