Depends on https://github.com/quotient-im/libQuotient/pull/695
Currently basic just to show a working implementation using RoomMember. Currently only the room event and search models are moved over. Will change everything else over once the dependent pr is complete.
- Have LinkPreviewers stored in NeoChatConnection so that they don't have to be reloaded everytime the MessageContentModel is refreshed
- This means the link is never changed (it will be swiched for a new previewer with the new link)
- LinkPreviewers are stored by URL so they can be re-used by any event with the same URL
BUG: 484927 (because the offending code is ripped out)
This adds previews for downloaded pdfs and code files.


Move the itinerary model representation to it's own component and instantiate from MessageComponentModel. This starts to lay some groundwork for previewing other files.
The aim is to be able to use separate delegate for things like codeblocks and quotes so that they can be styled differently.

For now everything should look identical. However this moves to using a model for the content of the message and is intended to lay the foundation for improved message content representation, e.g. splitting up a text message in multiple sections and using different delegates for things like code and quotes.