Add appdata for gtk4-demo

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

demos/gtk-demo/meson.build

@@ -131,3 +131,6 @@ install_data('gtk4-demo.desktop', install_dir: gtk_applicationsdir)
 # GSettings
 install_data('org.gtk.Demo.gschema.xml', install_dir: gtk_schemasdir)
 gnome.compile_schemas()
+
+# appdata
+install_data('org.gtk.Demo.appdata.xml', install_dir: gtk_appdatadir)

demos/gtk-demo/org.gtk.Demo.appdata.xml

@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop">
+  <id>org.gtk.Demo.desktop</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>LGPL-2.0+</project_license>
+  <name>GTK+ Demo</name>
+  <summary>Program to demonstrate GTK+ functions</summary>
+  <description>
+    <p>
+      GTK+ Demo is a collection of examples that demonstrate the major
+      features of the GTK+ toolkit.
+    </p>
+  </description>
+  <screenshots>
+    <screenshot>
+      <image>https://static.gnome.org/appdata/gtk4-demo/gtk-demo1.png</image>
+      <caption>The main window</caption>
+    </screenshot>
+    <screenshot>
+      <image>https://static.gnome.org/appdata/gtk4-demo/gtk-demo2.png</image>
+      <caption>An example</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>

meson.build

@@ -125,6 +125,7 @@ gtk_localedir = join_paths(gtk_prefix, get_option('localedir'))
 gtk_sysconfdir = join_paths(gtk_prefix, get_option('sysconfdir'))
 gtk_applicationsdir = join_paths(gtk_datadir, 'applications')
 gtk_schemasdir = join_paths(gtk_datadir, 'glib-2.0/schemas')
+gtk_appdatadir = join_paths(gtk_datadir, 'metainfo')
 
 cc = meson.get_compiler('c')