Fix #34846, Node->Ungroup menu entry is broken. The standard node group operators now don't require the node_type string property any more. They are limited to the Shader, Compositing and Texture node tree types and will pass through for other (pynodes) tree types. Associated node group types are hardcoded.

The original rationale for adding the node_type property was to allow node group operators work generically on any node group type automatically. The problem is that detecting the appropriate node group type and node tree type to use for a group depends on using a node base type. Due to the fact that RNA does not allow multiple inheritance (mixin classes) this is impossible to achieve if node types also have to use a base type such as ShaderNode, CompositingNode or TextureNode.

The idea is now to just "make it work" by limiting the node group operators to the standard tree types. For future pynodes we can implement these operators nicely in Python, which will allow pynodes to use mixin base classes or derive their own operator types and re-use the same keymapping.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56183 954f8c5b-7b00-dc11-b283-0030488c597c