Hey,
Is it posible to insert a "cancel"or "undo" button in the album manager ?
just use the "back" button of your browser (or "refresh"). Everything you do in album manager is done on the client (the browser side) by JavaScript, untill you hit the "Apply modifications" or choose another category, so the server doesn't keep a history of the things you did. Therefor, there can't be an undo function.
If you hit "Apply modifications", you're being prompted if you really want to apply the changes, that's where your cancel button is...
GauGau