application: Set default window icon to appid by default

Lots of newer apps that use their appid as their icon name don't set
window icons, since they aren't used in GNOME. Instead of setting it
manually in every app, just default to it.

Only set the icon if it exists in the icon theme.

Remove manually set default icons in the demo.

No tests as GtkApplication doesn't have any in the first place.

Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/7120

demos/gtk-demo/main.c

@@ -1056,8 +1056,6 @@ command_line (GApplication            *app,
 
   window = gtk_application_get_windows (GTK_APPLICATION (app))->data;
 
-  gtk_window_set_icon_name (GTK_WINDOW (window), "org.gtk.Demo4");
-
   if (name == NULL)
     goto out;
 

demos/node-editor/node-editor-application.c

@@ -223,8 +223,6 @@ node_editor_application_activate (GApplication *app)
   if (g_strcmp0 (PROFILE, "devel") == 0)
     gtk_widget_add_css_class (GTK_WIDGET (win), "devel");
 
-  gtk_window_set_icon_name (GTK_WINDOW (win), "org.gtk.gtk4.NodeEditor");
-
   gtk_window_present (GTK_WINDOW (win));
 }
 

demos/print-editor/print-editor.c

@@ -788,7 +788,6 @@ activate (GApplication *app)
   if (g_strcmp0 (PROFILE, "devel") == 0)
     gtk_widget_add_css_class (GTK_WIDGET (main_window), "devel");
 
-  gtk_window_set_icon_name (GTK_WINDOW (main_window), "org.gtk.PrintEditor4");
   gtk_window_set_default_size (GTK_WINDOW (main_window), 400, 600);
   gtk_application_window_set_show_menubar (GTK_APPLICATION_WINDOW (main_window), TRUE);
   update_title (GTK_WINDOW (main_window));

demos/widget-factory/widget-factory.c

@@ -2232,7 +2232,6 @@ activate (GApplication *app)
   if (g_strcmp0 (PROFILE, "devel") == 0)
     gtk_widget_add_css_class (GTK_WIDGET (window), "devel");
 
-  gtk_window_set_icon_name (window, "org.gtk.WidgetFactory4");
   gtk_application_add_window (GTK_APPLICATION (app), window);
   g_action_map_add_action_entries (G_ACTION_MAP (window),
                                    win_entries, G_N_ELEMENTS (win_entries),

gtk/gtkapplication.c

@@ -88,6 +88,10 @@
  * displays the shortcuts window, associate the item with the action
  * `win.show-help-overlay`.
  *
+ * `GtkApplication` will also automatically set the application id as the
+ * default window icon. Use [func@Gtk.Window.set_default_icon_name] or
+ * [property@Gtk.Window:icon-name] to override that behavior.
+ *
  * ## A simple application
  *
  * [A simple example](https://gitlab.gnome.org/GNOME/gtk/tree/main/examples/bp/bloatpad.c)
@@ -244,6 +248,23 @@ gtk_application_load_resources (GtkApplication *application)
   }
 }
 
+static void
+gtk_application_set_window_icon (GtkApplication *application)
+{
+  GtkIconTheme *default_theme;
+  const char *appid;
+
+  if (gtk_window_get_default_icon_name () != NULL)
+    return;
+
+  default_theme = gtk_icon_theme_get_for_display (gdk_display_get_default ());
+  appid = g_application_get_application_id (G_APPLICATION (application));
+
+  if (!gtk_icon_theme_has_icon (default_theme, appid))
+    return;
+
+  gtk_window_set_default_icon_name (appid);
+}
 
 static void
 gtk_application_startup (GApplication *g_application)
@@ -267,6 +288,7 @@ gtk_application_startup (GApplication *g_application)
   gtk_application_impl_startup (priv->impl, priv->register_session);
 
   gtk_application_load_resources (application);
+  gtk_application_set_window_icon (application);
 
   gdk_profiler_end_mark (before, "Application startup", NULL);
 }