Add explanatory text to the multi-delete dialog

New console users often confuse the Delete and the Delete * actions. The fact that you can delete multiple nodes in one go with the second option is not very clear. I think we can improve that by adding a bit of explanatory text at the top of the dialog.

Suggestion: "Select multiple nodes for deletion in the tree below."

Also changing the existing checkbox a bit can make it more clear. Put the checkbox before the text (like in the XML export dialog). Change the text to: "delete with child nodes. If you select nodes that have childnodes, but do not check this box, no action is taken when you click OK".