Plugin callbacks for Uri protocol

[TheSylence]

Allow a plugin to register a callback that is invoked when the application is called via buildnotification:// uri protocol.

This is needed to implement OAuth flow:

• The plugin registers a callback (e.g. buildnotification://plugin/oauth_callback)
• The plugin triggers an oauth
• The browser redirects to buildnotification://plugin/oauth_callback
• The plugin's callback is called

Things to consider:

• Allow any url registration or just for oauth and other simple cases?
  • Any registration would look like RegisterCallback( this, 'any/url' ) and register a buildnotifaction://any/url
  • By case would be something like RegisterOAuthCallback( this ) would register a buildnotifaction://oauth/plugin_name