[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/18797
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn