[BUGFIX] Clearing cache from toolbar fails in IE8

Fix for regression in #45595 that was introduced with #36364
IE8 does not pass the click event to the handler function and thus
raises a JavaScript error. The event can be fetched from the window
object in this case.

Fixes: #45595
Releases: 6.1, 6.0, 4.7, 4.6, 4.5

Change-Id: I9ccd5c8ae660a34e404122d9dd1c816909688b3e
Reviewed-on: https://review.typo3.org/18453
Reviewed-by: Mattias Nilsson
Tested-by: Mattias Nilsson
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Mario Rimann
Tested-by: Mario Rimann
Tested-by: Pascal Maechler
Reviewed-by: Stefan Neufeind
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn

typo3/js/clearcachemenu.js

@@ -48,6 +48,7 @@ var ClearCacheMenu = Class.create({
 				// observe all clicks on clear cache actions in the menu
 			$$('#clear-cache-actions-menu li a').each(function(element) {
 				$(element).onclick = function(event) {
+					event = event || window.event;
 					self.clearCache.call(self, event);
 					return false;
 				};