Grid, BlockGrid, Container: node constraints implemented
Buttons: improved, content restrictions
Callouts: cosmetics + node constraints
!!! Tabs: node types renamed!!! + node constraints + cosmetic improvements

* M12.Foundation:TabContentItem renamed to M12.Foundation:TabItem
* M12.Foundation:TabsAbstract renamed to M12.Foundation:Tabs
* M12.Foundation:TabsX removed: with Neos 1.2 interface + node constraints it's easy enough for editor to add desired amount of TabItems ad-hoc