Set baseNodeType for `_nodeType` property to limit number of allowed types to change to

[ryzy]

Each Neos node can be changed/migrated into another type. Currently we do not set the baseNodeType option for that _nodeType property (which has NodeTypeEditor). As an effect, each node can be migrated to any content node...

To make it more intuitive, we could set baseNodeType to some parent abstract node type, so the list is shorter and more sensible.