Investigate retrieving seed through the shell for electron app

Instead of having a `/wallet/seed` API enabled for the electron app, see if it is possible to safely run the skycoin-cli tool on the shell to retrieve the seed from a wallet (add a cli command if necessary) and reveal it to the user.

This will cause "show seed" to not work when running in the browser, but we can give instructions to the user on how to retrieve their seed from the files directly in that case, when they open the show seed panel.