Update some C# examples

- Rename members that have been renamed in Godot's C# API for 4.0.
- Change `delta` parameter type to `double`.
- Ensure parameters match base declaration.
- Follow our code style more closely.
- Other minor code fixes.

tutorials/plugins/editor/making_main_screen_plugins.rst

@@ -134,7 +134,7 @@ Add a script to the button like this:
     [Tool]
     public partial class PrintHello : Button
     {
-        public void OnPrintHelloPressed()
+        private void OnPrintHelloPressed()
         {
             GD.Print("Hello from the main screen plugin!");
         }