* Fix selector in subtree loading callback
  * Remove HTML inconsistencies between AJAX and plain PHP lists
  * Unify icon and CSS class switching in dw_tree and dw_mediamanager

  * Increase HTTP cache time since the resources are timestamped on request
    anyway
  * Check userscript.js only once for JS cache validation
  * Use cache class

Empty to, cc or bcc fields could lead to an Internal Server Error:
malformed header from script. Bad header=No recipient addresses found...

This ignores warnings because of the (intentionally) missing url
parameter for the constructor of SimplePie_File.