Rework targetsetuppage interface slightly

 * Remove PathChooser widget so that you need to use a proper dialog
   to select pathes. So typing into that PathChooser is no longer slow;-)

   The PathChooser does not provide a proper signal on when a user is done
   entering a directory, so it was not a good choice.

Tasknumber: QTCREATORBUG-1128
Reviewed-by: dt