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…
Hmm, I would rather prefer ``backend`` as ``admin`` for me doesn't makes clear enough that the interface is meant.

I've choosen ``disabled`` because enabled is the standard. If there's no setting in config you get ``null`` as value that's an equivalent of ``false``. So having ``enabled`` not set in config would mean the same as standard ``enabled: false`` to me.