Make EditorInterface accessible as a singleton

- EditorPlugin.get_editor_interface() is removed as redundant.

doc/classes/EditorInterface.xml

@@ -5,7 +5,16 @@
 	</brief_description>
 	<description>
 		[EditorInterface] gives you control over Godot editor's window. It allows customizing the window, saving and (re-)loading scenes, rendering mesh previews, inspecting and editing resources and objects, and provides access to [EditorSettings], [EditorFileSystem], [EditorResourcePreview], [ScriptEditor], the editor viewport, and information about scenes.
-		[b]Note:[/b] This class shouldn't be instantiated directly. Instead, access the singleton using [method EditorPlugin.get_editor_interface].
+		[b]Note:[/b] This class shouldn't be instantiated directly. Instead, access the singleton directly by its name.
+		[codeblocks]
+		[gdscript]
+		var editor_settings = EditorInterface.get_editor_settings()
+		[/gdscript]
+		[csharp]
+		// In C# you can access it via the static Singleton property.
+		EditorSettings settings = EditorInterface.Singleton.GetEditorSettings();
+		[/csharp]
+		[/codeblocks]
 	</description>
 	<tutorials>
 	</tutorials>