Support for custom UIA events

[ivan-danilov]

Registration of events works in CustomPatternSchemaBase. It is needed:

• to get custom event info via reflection in CustomPatternBase in the same way as with standalone UIA properties;
• provide mechanism to determine if active listeners of particular event exist (same as AutomationPeer.ListenerExists() does) and raising the event (see comment)
• write tests that validate implemented events are working and convenient either in WPF-based automation peer environment and in native case.

(I don't need this one right now, so contributions are welcome, if any)