StateEvent; EventContent::SimpleContent; event types refactoring

* StateEvent<> is a new class template for all state events. It provides a uniform interface to the state content, as well as a means to serialize the content back to JSON. In addition, StateEvent now parses the "prev_content" JSON object, so one can refer to the previous state now (a notable step to proper reflection of state changes in the displayed timeline in clients).
* EventContent::SimpleContent, together with StateEvent<>, forms a generalisation for simple state events, such as room name, topic, aliases etc. that boil down to a single key-value pair. DECLARE_SIMPLE_STATE_EVENT is a macro defined to streamline creation of events based on SimpleContent, providing API back-compatibility for events defined so far. As a result, a very concise simplestateevents.h replaces all those room*event.* files.
* Event/RoomEvent::fromJson() code is squeezed down to plain type lists passed to makeIfMatches() "chained factory" function template. TypeId is mandatory for an event type to be included into that factory.
* Event::toTimestamp() and Event::toStringList are completely superseded by respective fromJson<>() converters.

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