Add a get_output_node() method to the CompositorNodeTree python api

In a compositor node tree, the user can add many Composite nodes, but only one can be active as the final output.

There is currently no way in the python api to detect which one of these nodes is set as the render output.

In materials, the same situation is possible, but we have a get_output_node() method dedicated to identify the right output on the ShaderNodeTree class.

The idea is to port this function to the CompositorNodeTree class.