Scripting: Add a UI Layout for Node Headers

Some of the built-in nodes in blender have UI elements in their headers, but this control is not exposed to the python API.

There is already a UI layout for the node body, so there could be one for the header as well, which would be used through a draw_header function:

def draw_header(self, context, layout): layout.prop(context.scene, "my_property")

UI elements would be added the same as with any other draw function