Unity editor menu item for creating EditorWindow subclass. Place this file into 'Editor' folder.

CreateEditorWindowSubclassMenuItem.cs

using UnityEditor;
using System.CodeDom;
using System.IO;
using System.CodeDom.Compiler;

public class CreateEditorWindowSubclassMenuItem {
	[MenuItem("Assets/Create/File/EditorWindow Subclass...", false, 12)]
	private static void CreateEditorWindowClass() {
		var filePath = EditorUtility.SaveFilePanelInProject("Save file", "EditorWindowSubclass", "cs", "Enter class name");
		if (!string.IsNullOrEmpty(filePath)) {
			using (var streamWriter = new StreamWriter(File.Create(filePath))) {
				using (var tw = new IndentedTextWriter(streamWriter, "    ")) {
					var fileName = Path.GetFileNameWithoutExtension(filePath);
					var unit = BuildEditorWindowSubclassUnit(fileName);
					var provider = CodeDomProvider.CreateProvider("CSharp");
					provider.GenerateCodeFromCompileUnit(unit, tw, new CodeGeneratorOptions());
				}
			}
		}
		AssetDatabase.Refresh();
	}

	private static CodeCompileUnit BuildEditorWindowSubclassUnit(string className) {
		var unit = new CodeCompileUnit();

		var @namespace = new CodeNamespace();
		unit.Namespaces.Add(@namespace);

		@namespace.Imports.Add(new CodeNamespaceImport("UnityEngine"));
		@namespace.Imports.Add(new CodeNamespaceImport("UnityEditor"));
		@namespace.Imports.Add(new CodeNamespaceImport("System.Collections"));

		var declaration = new CodeTypeDeclaration { Name = className };
		@namespace.Types.Add(declaration);

		declaration.BaseTypes.Add(new CodeTypeReference { BaseType = "EditorWindow" });

		var methodAwake = new CodeMemberMethod {
			Name = "Init",
			Attributes = MemberAttributes.Static
		};
		methodAwake.Statements.Add(new CodeVariableDeclarationStatement(className, "window"));
		methodAwake.Statements.Add(
			new CodeAssignStatement(
				new CodeVariableReferenceExpression("window"),
				new CodeMethodInvokeExpression(
					new CodeMethodReferenceExpression(
						null,
						"CreateInstance",
						new CodeTypeReference(className)
					)
				)
			)
		);
		methodAwake.Statements.Add(
			new CodeExpressionStatement(
				new CodeMethodInvokeExpression(
					new CodeMethodReferenceExpression(
						new CodeVariableReferenceExpression("window"),
						"Show"
					)
				)
			)
		);

		declaration.Members.Add(methodAwake);
		declaration.Members.Add(new CodeMemberMethod { Name = "OnGUI" });

		return unit;
	}
}