endSessionDialog: Add extra strings for translation

Add two extra, currently unused strings in order to make the life easier
for translators and avoid breaking the upcoming string freezes.

https://bugzilla.gnome.org/show_bug.cgi?id=722898

js/ui/endSessionDialog.js

@@ -127,6 +127,8 @@ const restartInstallDialogContent = {
     showBatteryWarning: true,
     confirmButtons: [{ signal: 'ConfirmedReboot',
                        label:  C_("button", "Restart & Install") }],
+    unusedFutureButtonForTranslation: C_("button", "Install & Power Off"),
+    unusedFutureCheckBoxForTranslation: C_("checkbox", "Power off after updates are installed"),
     iconName: 'view-refresh-symbolic',
     iconStyleClass: 'end-session-dialog-shutdown-icon',
     showOtherSessions: true,