appIndicator: Correctly mark cached GIcon's as in use

The icon cache expects the icons to be marked as "inUse" as still valid,
however we were not setting this parameter correctly:

 - When setting it to true, we need to ensure that the cached icon that
   we got from the IconCache is actually the one we're using (as we may
   be using an equal object, but not the same instance).
 - When unsetting it, we need to read the actual Icon from the parent
   Gio.EmblemedIcon or Gio.Emblem.