Override window.PrivateBrowsingUtils.isWindowPrivate() for window.warnAboutClosingWindow()
(workaround for #53, part 6)
Fix possible memory leak: destroy window.privateTab on shutdown
Always check for last private tab closing
(workaround for #53, part 7)
Ignore forgetClosedTab() failure, if tab was moved to another window
(enhancement for #6, related to #36)
Check for last private tab closing in capturing phase to prevent errors from WindowsPreviewPerTab.jsm
(workaround for #53, part 8)