Add `Events::retain`

## What problem does this solve or what need does it fill?

Sometimes it's useful to not drain all events from `Events`, but by a specific predicate.

## What solution would you like?

There is [Vec::retain](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.retain) and it would be convenient to have something like this in `Events` too.

## What alternative(s) have you considered?

Other solutions to solve and/or work around the problem presented.

## Additional context

Many networking crates translate messages into events (like mine) and it would be convenient to be able to drop events from a particular client if it was disconnected from the server.