Add ability to remove private tabs from undo close history only after private browsing ends

[Infocatcher]

In addition for #6: we have secret extensions.privateTab.rememberClosedPrivateTabs preference to not remove private tabs from undo close history, so it's nice to have ability to cleanup undo close history after closing of last private tab/window.

Private data will be saved to the disk (since we can't alter built-in session restore mechanism), but this may be appropriate for some users.

[Infocatcher]

Summary:

• Set extensions.privateTab.rememberClosedPrivateTabs to true to not remove private tabs from undo close history.
• Additionally set extensions.privateTab.rememberClosedPrivateTabs.cleanup to 1 or 2 (see notes in defaults/preferences/prefs.js about the difference) to cleanup undo close history only after last private tab/window will be closed.

But be careful: undo close history with private tabs will be saved to the disk!
The data will be overwritten later, but can be (probably) restored.
With recent commits any private data shouldn't be saved to the disk.

[Infocatcher]

Preference renamed:
rememberClosedPrivateTabs.enableCleanup (boolean) -> rememberClosedPrivateTabs.cleanup (integer)
(see changes in defaults/preferences/prefs.js)

P.S. Summary updated.

[Infocatcher]

Note: private tabs aren't saved in undo close history in Firefox 29+:
https://bugzilla.mozilla.org/show_bug.cgi?id=899276