Add [NEW PAGE] page and [VSPACE] to Insert menu (#848)

* Add menu entries for inserting new page and vspace codes
* Update tests

tests/test_gui/test_gui_theme.py

@@ -36,6 +36,7 @@ def testGuiTheme_Main(qtbot, monkeypatch, nwMinimal, tmpDir):
     """
     # Block message box
     monkeypatch.setattr(QMessageBox, "question", lambda *a: QMessageBox.Yes)
+    monkeypatch.setattr(QMessageBox, "warning", lambda *a: QMessageBox.Yes)
 
     nwGUI = nw.main(["--testmode", "--config=%s" % nwMinimal, "--data=%s" % tmpDir, nwMinimal])
     qtbot.addWidget(nwGUI)