add backspace to QKeySequence::Delete on Mac OSX

In Qt 4.3.3 documentation it's stated that QKeySequence::Delete
corresponds to Delete and Meta+Delete shortcuts. But native OSX
applications also use Backspace to remove items from containers.

Some examples:

• Opening Preview.app with several images, then selecting the right
  pane with the individual images
• iTunes, when some track is selected
• Address Book.app, when a contact is selected
• Finder, when file is selected

It's basically this way because on laptop Mac keyboards there is no
easily accessible Delete key at all, all's left is Backspace.

It would be nice if Backspace was added to the list of the shortcuts.