Store passwords securely

[Venemo]

Currently communi stores all settings (including passwords) without any security. It would cost minimal effort to deserialize the QByteArray which is currently written into the config file.

Idea from #107: we could maybe use QtKeychain.

QtKeychain is a Qt API to store passwords and other secret data securely. How the data is stored depends on the platform:

• Mac OS X: Passwords are stored in the OS X Keychain.
• Linux/Unix: If running, GNOME Keyring is used, otherwise qtkeychain tries to use KWallet (via D-Bus), if available.
• Windows: By default, the Windows Credential Store is used (requires Windows 7 or newer). Pass -DUSE_CREDENTIAL_STORE=OFF to cmake use disable it. If disabled, QtKeychain uses the Windows API function CryptProtectData to encrypt the password with the user's logon credentials. The encrypted data is then persisted via QSettings.