RoomMessageEvent: refactored boilerplate code; added support of HTML-formatted messages

[KitsuneRal]

This breaks compatibility and is massive in itself, hence a PR.

While a lot of code has been moved around, it boils down to the following 5 things:

• {Type}EventContent classes have moved to a dedicated namespace (MessageEventContent), to save on typestrokes.
• The contents of these classes have been deduplicated by moving out common data into common parents; a trick with a template inheriting from its own parameter allowed to make a factory for event media content types
• Parsing JSON has been deduplicated respectively. Fortunately, the spec is (almost) systematic in keys naming for different message types.
• body member variable is now plainBody and lives in RoomMessageEvent, right next to msgtype. A primitive client can use these two and live in plain text - without looking at RoomMessageEvent::content() at all.
• A separate class for text messages has been made, to store formatted text from formatted_body. The same class also can read from body and try to deduce the content type using Qt facilities. This may look controversial but in the end you always have RoomMessageEvent::plainBody() for fallback.

[KitsuneRal]

To follow up with necessary updates on the client side (actually, the second item can be covered be a "compatibility" addition to RoomMessageEvent if you consider necessary - but I'd rather push clients to the new lib interface):

• for plain text messages, just use RoomMessageEvent::body(), as you should have been doing by now;
• for media content, instead of MessageEventContent derivatives, use respective classes from MessageEventContent namespace (such as Image);
• to get HTML text messages (in particular, those produced by Vector), use MessageEventContent::Text and its body field (with an accessor in RoomMessageEvent being a fallback).