Showing Telegram's notifications on the lock screen

## I'm submitting a…

[x] Bug

[ ] Feature Request

[ ] Other (Please describe in detail)

## Current Behavior

i3lock shows telegram's notifications(and probably other apps as well) while its locked, which might contain sensitive information.

## Expected Behavior

Not showing anything from outside lock screen.

## Reproduction Instructions

1. open telegram desktop
2. lock your device
3. send a message to your telegram account, and you'll get the message on the lock screen

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

i3lock version:  2.14.1