Set window class name (xclass) in test/editor

... to avoid side effects under Windows where the default would
prevent entering emoji's with the emoji palette if the WNDCLASS
begins with "edit" (conflict with a system WNDCLASS).

This is also documented in the respective tutorial chapter.

documentation/src/editor.dox

@@ -57,6 +57,7 @@ Fl_Double_Window *app_window = NULL;
 
 void tut1_build_app_window() {
   app_window = new Fl_Double_Window(640, 480, "FLTK Editor");
+  app_window->xclass("fl_editor");
 }
 
 int main (int argc, char **argv) {
@@ -66,6 +67,11 @@ int main (int argc, char **argv) {
 }
 \endcode
 
+Note: setting the xclass() of the window is not required but it is considered
+  good style. In this case it works around a known side effect under \b Windows
+  where leaving the default ("editor.exe") would prevent entering emojis with
+  the emoji palette. You can ignore this for now in this tutorial.
+
 Passing `argc` and `argv` to `Fl_Double_Window::show()` allows FLTK to parse
 command line options, providing the user with the ability to change the
 color or graphical scheme of the editor at launch time.

test/editor.cxx

@@ -4,7 +4,7 @@
 // This program is described in chapter "Designing a Simple Text Editor"
 // of the FLTK Programmer's Guide.
 //
-// Copyright 1998-2025 by Bill Spitzak and others.
+// Copyright 1998-2026 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -31,6 +31,7 @@ Fl_Double_Window *app_window = NULL;
 
 void tut1_build_app_window() {
   app_window = new Fl_Double_Window(640, 480, "FLTK Editor");
+  app_window->xclass("fl_editor");
 }
 
 #endif