shift-delete: Delete immediately

Hi,

I made a small patch that implements 'shift-delete': Delete messages immediately without moving them to the Trash folder. This helps people who are over quota and therefore cannot copy any messages to the Trash folder.

The key_press method of rcube_list_widget simply sets the shiftkey property whenever a key is pressed.

It works only with the shift key and the del key, not with the delete button. I haven't looked at that yet, don't even know if it's possible because my browser seems to handle shift-click it's own way.

Please comment.

Robin