Rework typing notifications

Previously, the bot only had rudimentary typing notification support.

It used to send a single notification when starting a long task
and did not bother with notifications anymore.
By default matrix-rust-sdk gives these notifications a validity of 4
seconds, so it would expire shortly. If the bot takes longer to respond,
you'd see the typing notification expire and wonder if a response is
coming.

Another edge case is the bot sending an answer quicker and the typing
notice still being on. Some clients (like element-web) seem to hide the
typing notice when a new message comes, so they don't experience this as
problematic.

The reworked typing notification system should be robust:

- typing notices are sent continuously, until the bot finishes doing
  work
- if the bot is performing multiple actions in a room (even for
  different people), typing notices would continue to be sent until the
  bot becomes idle
- as soon as the bot becomes idle, a "not typing anymore" notice is sent
  to clear the state

Cargo.toml

@@ -22,7 +22,7 @@ base64 = "0.22.*"
 # We'd rather not depend on this, but we cannot use the ruma-events EventContent macro without it.
 matrix-sdk = { version = "0.7.1", default-features = false }
 mxidwc = "1.0.*"
-mxlink = "1.1.*"
+mxlink = "1.2.*"
 etke_openai_api_rust = "0.1.*"
 quick_cache = "0.6.*"
 regex = "1.10.*"