Implement Fl_Window::xclass(const char*) for the Wayland platform (#1391).

src/Fl_Window.cxx

@@ -241,9 +241,12 @@ void Fl_Window::default_xclass(const char *xc)
   and the second one if the first is 'x'.  Thus "foo" turns into "foo, Foo",
   and "xprog.1" turns into "xprog, XProg".
 
+  <I>Under Wayland</I>, this is used to set the "application identifier" of toplevel,
+  bordered windows. If not set, the executable name is used.
+
   <I>Under Microsoft Windows</I>, this string is used as the name of the
-  WNDCLASS structure, though it is not clear if this can have any
-  visible effect.
+  WNDCLASS structure. The user should avoid names that may collide with
+  system-reserved window class names (e.g., anything beginning with "edit").
 
   \since FLTK 1.3 the passed string is copied. You can use a local
   variable or free the string immediately after this call. Note that

src/drivers/Wayland/Fl_Wayland_Window_Driver.cxx

@@ -1458,7 +1458,8 @@ void Fl_Wayland_Window_Driver::makeWindow()
     new_window->frame = libdecor_decorate(scr_driver->libdecor_context, new_window->wl_surface,
                                           &libdecor_frame_iface, new_window);
     // appears in the Gnome desktop menu bar
-    libdecor_frame_set_app_id(new_window->frame, get_prog_name());
+    const char *appid = pWindow->xclass() ? pWindow->xclass() : get_prog_name();
+    libdecor_frame_set_app_id(new_window->frame, appid);
     libdecor_frame_set_title(new_window->frame, pWindow->label()?pWindow->label():"");
     if (!is_resizable()) {
       libdecor_frame_unset_capabilities(new_window->frame, LIBDECOR_ACTION_RESIZE);