[Enhancement] Security option in Settings for hide mnemonic and timeout shutdown

[tadeubas]

Some users want to be more secure when loading a wallet by not showing the mnemonic and encrypting the mnemonic menus after loading. Others want an automatic shutdown after some inactivity.

So the idea is to create another menu entry in Settings called Security and add these two entries:

• Hide mnemonic
• Timeout shutdown

When enabled, hide mnemonic will hide the Mnemonic and Encrypt Mnemonic entries from the Main Menu (only for loaded wallets, not for newly created ones).

Timeout shutdown allows the user set a time in minutes from 0 (disabled) to 60. It should shutdown the device in any screen if idle for the configured time.