Add AppImageHub in-app catalog browser with search, categories, and install

data/app.driftwood.Driftwood.gschema.xml

@@ -124,6 +124,11 @@
       <summary>Security notification threshold</summary>
       <description>Minimum CVE severity for desktop notifications: critical, high, medium, or low.</description>
     </key>
+    <key name="catalog-last-refreshed" type="s">
+      <default>''</default>
+      <summary>Catalog last refreshed</summary>
+      <description>ISO timestamp of the last catalog refresh.</description>
+    </key>
     <key name="watch-removable-media" type="b">
       <default>false</default>
       <summary>Watch removable media</summary>