'Backup accounts' menu-item non-reactive if no accounts yet created

[luclu]

As geth will create the keystore folder on demand the account backup menu-item will be non reactive on the first screens of the on-boarding:

There are two reasons for the availability of the menu-item:

1. to backup accounts
2. to import web3-secret-storage files

As for 2. the current importer only supports presale-wallets, though #997 will extend its functionality to cover web3-secret-storage files.

Following ways are feasible:

1. unload the menu-item during the on-boarding
2. disable it (and add some hint to it) during the on-boarding
3. leave it as is and let it create the folder if not available
4. leave it as is and let it open the closest available parent folder

Possibility 3 and 4 might not be node agnostic and thus prevent the abstraction of the backend nodes as #1064 tries to initiate. 😈
(..for sure this can be case depending if the node name is known to Mist considering the low number of nodes.)