demos: Generate appdata

We don't maintain a release history here, but at least
we can put in the right version and date for the current
build.

Fixes: #3993

demos/icon-browser/org.gtk.IconBrowser4.appdata.xml.in

@@ -0,0 +1,39 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop">
+  <id>org.gtk.IconBrowser4.desktop</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>LGPL-2.0+</project_license>
+  <name>GTK Icon Browser</name>
+  <summary>Program to browse themed icons</summary>
+  <description>
+    <p>
+      GTK Icon Browser is a simple application to show themed icons that
+      are available on the system.
+    </p>
+  </description>
+  <screenshots>
+    <screenshot>
+      <image>https://static.gnome.org/appdata/gtk4-icon-browser/gtk-icon-browser1.png</image>
+      <caption>Icon Browser</caption>
+    </screenshot>
+    <screenshot>
+      <image>https://static.gnome.org/appdata/gtk4-icon-browser/gtk-icon-browser2.png</image>
+      <caption>Search</caption>
+    </screenshot>
+  </screenshots>
+  <kudos>
+    <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>
+  <releases>
+    <release version="@BUILD_VERSION@" date="@BUILD_DATE@">
+      <description>
+        <p>A new build of GTK.</p>
+      </description>
+    </release>
+  </releases>
+</component>