Add option to lock the app

[germefur]

it will be nicer if telegrand be blocked like telegram desktop does

[melix99]

Just leaving this here for future discussions about this: we could include an option to encrypt the db using an user password. This has some drawbacks, so it needs to be discussed.

More information about this: https://github.com/tdlib/td/issues/188#issuecomment-379536139