Wrap creation of Playdar api in libtomahawk-playdarapi 

* Qxt is now only used in libtomahawk-playdarapi and not required in any
  other target.
Add some comments to tomahawk.js
Move ACLRegistry to network/acl/ as it takes care of networking-access
Add WeakObjectList helper class
Encapsulate ACL requests/decisions to move logic out of Connection