This prepares for the keybinding hints PR.

To reproduce, press `ctrl-b` in a GPUI app with `ctrl-b h` and `ctrl-b
j` bindings, then call `window.blur()`. The live pending state clears,
but observers still report `ctrl-b`.

`Window::blur` now clears pending input and notifies observers after the
current effect cycle. I don't think we currently exercise this path in
Zed, and I was only able to reproduce it with an example GPUI app.

Release Notes:

- N/A