Option to disable stack usage

You now can disable the usage of the stack. If set to ``false`` the stack and correspondening buttons are removed from the user interface.

Is path ``general/interface/stack/disable`` good named? For me it's ok but we have already quite some not that lucky naming in config. So some discussion before adding further ones perhaps isn't a bad idea. :-)

Eg. using ``gui`` instead of ``interface`` comes to mind…
Agreed on `backend`.  I thought `interface` was too ambiguous.

With `enabled` versus `disabled`, we can set defaults in Config.php anyway if needed.  My thought is that using the key `enabled` is more obvious to a non-technically savvy user