Send read receipts based on timeline scroll position

[alanpoon]

Rework of #124

1. Removed lazy_static
2. Replaced thread with makepad timer
3. Removed de-duplicate logic when sending read receipt
4. Removed un-neccessary SignalToUI
5. Moved fields into TimelineUiState

Closes #86

[alanpoon]

PR for #86

[kevinaboos]

thanks for the PR!

One request -- can you follow standard whitespace conventions, like a space after a comma? Without that, it's hard to read the code easily.

I left a few detailed comments.

[kevinaboos]

I did some minor cleanup just to improve readability, so take a look at those minor changes to see what sort of code style and whitespace conventions we're looking for. The logic for tracking which events have been read is still needlessly complicated, and also rather inefficient, especially with the repeated population of big hashmaps and subsequent iteration over them.
I still believe that all you really need to do is start a 5-second timeout timer whenever scrolling stops, and then after the timer has reached its timeout deadline, send the read receipts for the last visible event.

That being said, your approach technically works despite being overly complicated, so in the interest of time I'm going to accept and merge it in, and we'll save other improvements for the future.

I left a few comments that mostly target C vs Rust coding styles and inefficiencies scattered throughout.

Please submit another PR to clean things up in the future.