Support transparency (RGBA) in i3 for border, background, indicator and child_border

## I'm submitting a…

[ ] Bug

[x] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

## Current Behavior
Transparency is currently not supported in i3. There is even a _bug_ report (#3932) where someone is wondering why his configuration does not work.

## Desired Behavior
If would be great if i3 would support transparency for [border, background, indicator and child_border](https://i3wm.org/docs/userguide.html#_changing_colors) colors, like it was implemented for i3bar some time ago (#3727).

This way one could even resemble the behavior of i3-gaps (to a certain degree) as long as it has not been merged into i3 (#3724).

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

i3 version: 

$ i3 --moreversion

Binary i3 version:  4.18 (2020-02-17) © 2009 Michael Stapelberg and contributors

Running i3 version: 4.18 (2020-02-17) (pid 2370)o abort…)

Loaded i3 config: /tmp/i3.1.config-5YMLQKYRZ6 (Last modified: Mon 21 Sep 2020 02:26:19 PM CEST, 686 seconds ago)


The i3 binary you just called: /usr/bin/i3

The i3 binary you are running: /usr/bin/i3

Linux Distribution & Version:

$ uname -a

Linux laptop 5.8.0-1-amd64 #1 SMP Debian 5.8.7-1 (2020-09-05) x86_64 GNU/Linux

$ picom --version

v8