Fix how we find existing frames for the purpose of form target= submission.
This fixes file upload for drupal 6/jquery.form --- its submit was incorrectly blocked as a popup attempt

As we refactored this method, also use it for window.open, fixing us blocking opens to _self along the way.

While testing this, I noticed that we can crash konq if we do window.open and one of the frames doesn't yet have a part yet.

Added a workaround suggested by dfaure for now, as the proper fix is too risky and should be done on unstable branch only.