Add old id to appdata file

data/org.gajim.Gajim.appdata.xml.in

 <?xml version="1.0" encoding="UTF-8"?>
 <component type="desktop-application">
   <id>org.gajim.Gajim.desktop</id>
+  <provides>
+    <id>gajim.desktop</id>
+  </provides>
   <metadata_license>CC-BY-SA-3.0</metadata_license>
   <project_license>GPL-3.0</project_license>
   <name>Gajim</name>