[Feature request] Set border color for sticky windows

## I'm submitting a…

[ ] Bug

[x] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

## Current Behavior

There is no indication that a window is sticky or not, it would be very convenient if it was possible to set a border color for sticky windowses

## Desired Behavior

Ability to set border color for sticky windowses

## Impact

- [x] This feature requires new configuration and/or commands

Maybe it could have its own color config to go along with the already existing color changing configuration
```
# class border backgr. text indicator child_border
client.sticky #000000 #0c0c0c #ffffff #000000 #0c0c0c
```
Or if there is an existing (hacky-ish) way to do such thing?

## Environment

Output of `i3 --moreversion 2>&-`:

i3 version: 4.21.1-1

- Linux Distribution & Version: Arch Linux x86_64 (Linux arch 6.0.8-zen1-1-zen)

- Are you using a compositor (e.g., xcompmgr or compton): Compton