Add waitForDialog(onOpen, button) support function

Waits for an alert or confirmation dialog to open and closes it
automatically, optionally after running onOpen(dialog) to check its
contents (e.g., with dialog.document.documentElement.textContent) and
optionally clicking a button other than 'accept' (e.g., 'cancel',
extra1').

Supports delayed accept buttons

test/tests/preferences_searchTest.js

@@ -7,10 +7,7 @@ describe("Search Preferences", function () {
 				action: 'pdftools-install'
 			});
 				// Wait for confirmation dialog
-			yield waitForWindow("chrome://global/content/commonDialog.xul", function (dialog) {
-				// Accept confirmation dialog
-				dialog.document.documentElement.acceptDialog();
-			});
+			yield waitForDialog();
 			
 			// Wait for install to finish
 			yield waitForCallback(function() {