Cleanup: VSE: Replace redundant "Change Effect Input" with "Swap Inputs"

The `SEQUENCER_OT_change_effect_input` operator does exactly the same thing
as `SEQUENCER_OT_swap_inputs` . "Change effect input" in the
"Effect Strip" context menu also seems bugged in recent versions.

Since "Swap Inputs" already has a keybind and the name itself is closer
to its intended use, just use that operator over the other one.

Also poll "Swap Inputs" so it is grayed out if the effect does not take
two inputs.

Pull Request: https://projects.blender.org/blender/blender/pulls/139615

scripts/startup/bl_ui/space_sequencer.py

@@ -656,8 +656,8 @@ class SEQUENCER_MT_change(Menu):
             del bpy_data_scenes_len
 
         layout.operator_context = 'INVOKE_DEFAULT'
-        layout.operator("sequencer.change_effect_input")
         layout.menu("SEQUENCER_MT_strip_effect_change")
+        layout.operator("sequencer.swap_inputs")
         props = layout.operator("sequencer.change_path", text="Path/Files")
 
         if strip:
@@ -999,7 +999,6 @@ class SEQUENCER_MT_strip_effect(Menu):
     def draw(self, _context):
         layout = self.layout
 
-        layout.operator("sequencer.change_effect_input")
         layout.menu("SEQUENCER_MT_strip_effect_change")
         layout.operator("sequencer.reassign_inputs")
         layout.operator("sequencer.swap_inputs")