FR: Emit a `lock` event on lock

## I'm submitting a…

[ ] Bug

[x] Feature Request

[ ] Other (Please describe in detail)

## Current Behavior

It seems like neither `i3` nor `i3lock` emit a `lock` event.

## Expected Behavior

It would be great if `i3lock` / `i3` could emit a `lock` event on screen lock that would allow other applications to capture and act upon it.

## Environment
Output of `i3lock --version`:

i3lock: version 2.11.1 © 2010 Michael Stapelberg

```sh
$ uname -a
Linux arch-usb 5.2.0-arch2-1-ARCH #1 SMP PREEMPT Mon Jul 8 18:18:54 UTC 2019 x86_64 GNU/Linux
```

## Related

This originally came from https://github.com/keepassxreboot/keepassxc/issues/3339 - a password manager could lock it's database on screenlock, but that currently doesn't seem possible with `i3` / `i3lock`, since there's no `lock` event (correct me if I'm wrong).

I'm sure there'd be more use cases:)