Undo button in snackbar.

**Is your feature request related to a problem? Please describe.**
There is no way to undo our actions performed. For example suppose a user by mistake deletes a torrent download so he will not be any chance to undo his action. Instead he should be given a chance for some seconds to do it.

**Describe the solution you'd like**
Undo button can be implemented in the snackbar instead of dismiss button to undo the action performed.