Ported over from 61365dfbf3 in the `gui` branch. This will allow
adding our own events, most notably an idle timer event (useful
for adding debounced input in [dynamic pickers][1] used by interactive
global search and workspace symbols).
[1]: https://github.com/helix-editor/helix/pull/3110
Co-authored-by: Blaž Hrastnik <blaz@mxxn.io>
634b6d455f