`SyncDisplay` is not truly concurrent

[notgull]

SyncDisplay is essentially a wrapper around Mutex<BasicDisplay>, which means that it cannot send requests while reading an event. This could be solved by splitting BasicDisplay into a "state" part and an "I/O" part, and then further split I/O into a read/write part. That way, only one of those has to be locked at a time, allowing for concurrency. BasicDisplay would consist of "state" and "I/O" put together, while CellDisplay and SyncDisplay would wrap them using RefCell and Mutex, respectively.