New hook 'AfterFinalPageOutput' which is called at the end of OutputPage::output()

New hook 'AfterFinalPageOutput' which is called at the end of OutputPage::output()

Called  before final ob_end_flush() which will send the buffered output to the client. This allows for last-minute modification of the output within the buffer by using ob_get_clean().

- patch-set2: moved the hook before OutputPage::sendCacheControl() is called, as suggested by Reedy.

Change-Id: I264a37ac74e0c18ff141f82156e669ece3212c2d