Implement account management designs

Implement the nice designs from @tobias.bernard https://gitlab.gnome.org/Teams/Design/app-mockups/-/blob/master/calls/voip-accounts.png

This is composed of the following tasks:

• Add a CallsCredentials class, which is a dumb container holding credentials (user, password, host, etc)
• Add a ProvidesAccountsInterface which is implemented by the CallsSipProvider (so that the UI knows that we can add accounts to it
• Add a OnlineAccountsInterface which is implemented by CallsSipOrigin to allow going online/offline, watching for state changes with respect to authentication and similar
• Do the plumbing work for the above items (at this point we would still be using the .ini config)
• Have an account manager, managing available accounts and allowing to save and load credentials in the keyring (using libsecret?)
• Add UI according to the designs