appdata: State hardware support the old way

This declares which hardware the application supports using the old
custom tags, as the news ones aren't used widely enough yet.

desktop/org.gnome.Geary.appdata.xml.in.in

@@ -260,4 +260,9 @@
     <display_length compare="ge">360</display_length>
   </requires>
 
+  <custom>
+    <value key="Purism::form_factor">workstation</value>
+    <value key="Purism::form_factor">mobile</value>
+  </custom>
+
 </component>

desktop/org.gnome.Geary.desktop.in.in

@@ -16,6 +16,7 @@ StartupNotify=true
 DBusActivatable=true
 Actions=compose;new-window
 X-GNOME-UsesNotifications=true
+# Translators: Do NOT translate or transliterate this text (these are enum types)!
 X-Purism-FormFactor=Workstation;Mobile;
 
 [Desktop Action compose]