Add secure deletion (shreding)

[rgimad]

• Add an option that makes all file deletion operations secure: firstly rewrite file contents with some zero/random data (by some algorithm, there're a bunch of them) and only after that delete. Option would look like this:

• Also, add this option to the CLI version

• Also, related to secure deletion, it would be very useful to implement shreding free space on selected disk. For this feature there is necessary to make a menu that shows all the disk in the system that we can shred free space on.