Implement ISecureStorage for all platforms

[OAuthority]

AWB stores sensitive information in the Windows Registry; this is obviously not an option for macOS and Linux -- each of these has their own alternatives, however.

There is no working/up-to-date wrapper around all of these storage mechanisms; there is KeySharp, although it doesn't seem to be that up-to-date. I have created an interface, ISecureStorage which needs to be extended to provide platform specific ways to access the storage mechanism.

• Windows -- Windows Credential Manager
• macOS -- MacOSKeychain.cs (implemented in 753c9a5)
• Linux -- Libsecret