Don't crash on drop in frames. Same deal as <a href="http://websvn.kde.org/?rev=739370">r739370</a>;, #151836 ---
this was playing ping-pong-with Qt with drop events, trying to propagate them up, and Qt sending them back down because it doesn't match its internal DND state. Just as in khtmlimagepart, just let Qt bubble them itself.

This makes behavior match KDE3.5.x --- one can drop things from different documents here, and that'll affect the document (one can't DND from one frame to another, but that was the case in 3.5.x, too, and it's probably the right thing to do..)