Provide as-before-event room context for a given timeline item

[KitsuneRal]

This is 'point of reference' for all troubles concerned with the fact that the library only knows of a single room state - the latest one. We need to make a kind of RoomView class that would allow to render events in the context of the room, as it were immediately before the event.

[KitsuneRal]

After all the write-up in #432 I have just found this issue. Although #432 is almost a duplicate, this one expects a bit more convenience for clients - and is harder to do. Therefore this one is planned for some later time (and maybe after closing #309), while #432 is more pressing but describes a minimal implementation specifically for 0.7.