Compositor: Redesign File Output node

This patch redesigns the File Output node to provide better UX and UI.
This is mainly achieved by allowing the user to create inputs by
dragging into an Extend socket and adjust existing inputs using the
familiar UI list design available in Blender. Additionally, various UI
changes were done:

- The Use Node Format option was renamed to Override Node Format for
  clarity.
- Socket types are now fixed and do not change as new links are made,
  allowing users to specify the exact output type and employ implicit
  conversion if needed.
- The distinction between images and Multi-Layer EXR was made clearer.
- Final output paths are drawn in the UI to remove guess work.
- The Base Path was split into a Directory and a File Name.
- Panels were added to group options, include a panel for the node
  format, items, and item formats.

Pull Request: https://projects.blender.org/blender/blender/pulls/141091

tests/python/compositor_file_output_tests.py

@@ -197,16 +197,17 @@ class FileOutputTest(unittest.TestCase):
         self.assertTrue(ok)
 
     def run_test_script(self, blendfile, curr_out_dir):
-        def set_basepath(node_tree, base_path):
+        def set_directory(node_tree, base_path):
             for node in node_tree.nodes:
                 if node.type == 'OUTPUT_FILE':
-                    node.base_path = f'{curr_out_dir}/'
+                    node.directory = f'{curr_out_dir}/'
+                    node.file_name = ""
                 elif node.type == 'GROUP' and node.node_tree:
-                    set_basepath(node.node_tree, base_path)
+                    set_directory(node.node_tree, base_path)
 
         bpy.ops.wm.open_mainfile(filepath=blendfile)
         # Set output directory for all existing file output nodes.
-        set_basepath(bpy.data.scenes[0].compositing_node_group, f'{curr_out_dir}/')
+        set_directory(bpy.data.scenes[0].compositing_node_group, f'{curr_out_dir}/')
         bpy.data.scenes[0].render.compositor_device = f'{self.execution_device}'
         bpy.ops.render.render()