demos: Add xmlns to appstream metainfo files

Pixiewood expects well-formed XML files, whose tags match the AppStream
namespace. Note that AppStream itself does not do this, so picking a
prefix for the namespace is fine for pixiewood but not AppStream.

demos/gtk-demo/org.gtk.Demo4.appdata.xml.in

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<component type="desktop">
+<component xmlns="https://specifications.freedesktop.org/metainfo/1.0" type="desktop">
   <id>org.gtk.Demo4</id>
   <launchable type="desktop-id">org.gtk.Demo4.desktop</launchable>
   <metadata_license>CC0-1.0</metadata_license>

demos/node-editor/org.gtk.gtk4.NodeEditor.appdata.xml.in

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<component type="desktop">
+<component xmlns="https://specifications.freedesktop.org/metainfo/1.0" type="desktop">
   <id>org.gtk.gtk4.NodeEditor</id>
   <launchable type="desktop-id">org.gtk.gtk4.NodeEditor.desktop</launchable>
   <metadata_license>CC0-1.0</metadata_license>

demos/print-editor/org.gtk.PrintEditor4.appdata.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<component type="desktop">
+<component xmlns="https://specifications.freedesktop.org/metainfo/1.0" type="desktop">
   <id>org.gtk.PrintEditor4</id>
   <launchable type="desktop-id">org.gtk.PrintEditor4.desktop</launchable>
   <metadata_license>CC0-1.0</metadata_license>

demos/widget-factory/org.gtk.WidgetFactory4.appdata.xml.in

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<component type="desktop">
+<component xmlns="https://specifications.freedesktop.org/metainfo/1.0" type="desktop">
   <id>org.gtk.WidgetFactory4</id>
   <launchable type="desktop-id">org.gtk.WidgetFactory4.desktop</launchable>
   <metadata_license>CC0-1.0</metadata_license>