improvements for password handling

- do not enforce password policy, only requires password to be existent (i.e. >0)
- added possibility to skip password protection
- automatically unlocks database if no password is set
improvements and enhancements for setting and entering passwords

- setting a password is now optional and informs user about iOS Data Protection
- do not enforce a password policy besides length > 0
- do not display entropy to user
- providing a switch to choose between PIN with numeric keyboard or alphanumeric passphrase
- store setting in UserDefaults to skip password entry (no password) or display numeric keyboard (PIN)