Provide a way to set a title bar icon for the Preferences dialog

[selundqma]

I miss a way to set a custom icon for the Preferences dialog in the class E4PreferencesHandler.

      // Can display the standard dialog.
      PreferenceDialog dialog = new PreferenceDialog(shell, pm);
      dialog.create();
      dialog.getTreeViewer().setComparator(new ViewerComparator());
      dialog.getTreeViewer().expandAll();
      dialog.getShell().setImage(<Image object containing custom icon>);
      dialog.open();

Right now you get a plain and kind of boring standard icon in the top left corner of the dialog.