Add appdata for gtk4-widget-factory

This will let us build a better flatpak for gtk4-widget-factory.

demos/widget-factory/meson.build

@@ -24,3 +24,6 @@ foreach icon_size : ['16x16', '22x22', '24x24', '32x32', '48x48', '256x256', ]
   install_data('data/@0@/gtk4-widget-factory-symbolic.symbolic.png'.format(icon_size),
                install_dir: icons_dir)
 endforeach
+
+# appdata
+install_data('org.gtk.WidgetFactory.appdata.xml', install_dir: gtk_appdatadir)

demos/widget-factory/org.gtk.WidgetFactory.appdata.xml

@@ -0,0 +1,34 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop">
+  <id>org.gtk.WidgetFactory.desktop</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>LGPL-2.0+</project_license>
+  <name>GTK+ Widget Factory</name>
+  <summary>Program to demonstrate GTK+ functions</summary>
+  <description>
+    <p>
+      GTK+ Widget Factory is a showcase of GTK+ widgets. It was
+      originally created to help theme authors test their creations
+      for completeness.
+    </p>
+  </description>
+  <screenshots>
+    <screenshot>
+      <image>https://static.gnome.org/appdata/gtk4-widget-factory/gtk-widget-factory1.png</image>
+      <caption>Page 1</caption>
+    </screenshot>
+    <screenshot>
+      <image>https://static.gnome.org/appdata/gtk4-widget-factory/gtk-widget-factory2.png</image>
+      <caption>Page 3</caption>
+    </screenshot>
+  </screenshots>
+  <kudos>
+    <kudo>AppMenu</kudo>
+    <kudo>HiDpiIcon</kudo>
+    <kudo>ModernToolkit</kudo>
+  </kudos>
+  <url type="homepage">https://www.gtk.org</url>
+  <translation type="gettext">gtk-4.0</translation>
+  <update_contact>matthias.clasen_at_gmail.com</update_contact>
+  <developer_name>Matthias Clasen and others</developer_name>
+</component>