This adds a button to utilize the domain isolation to get a new circuit for
the current tab only.

The private browsing pref doesn't actually take effect until restart..

We only hide this for now in case we want to bring it back as a secondary
tab after everyone cries about it. It also simplifies the code changes to
simply hide it.

Also clean up the associated Torbutton menu and strings for clarity.

Also update the string so it is more clear that it is a secondary
dialog, once we do get it working again.

Patch originally by Saint, but in this version we keep the actual dialog in
case we want to move it somewhere else later.

Might as well standardize on this, rather than going back and forth.

Always display a "Check for Tor Browser Update..." menu item (to avoid
localization issues, the menu item label entity was not renamed).  When the
menu item is chosen, open the Update Service prompt window to perform the
update check.

We are leaving custom mode now if the user manages it to undo all
security slider related preference changes manually. If, on the other
hand, the user changes a relevant preference while being in one of the
four regular slider modes we don't bother checking all preferences and
switch to the custom mode immediately.