build(deps): use submodules for libcec

[ReenigneArcher]

Description

• Add libcec as a submodule (at last released version). I confirmed the cecloader.h file is identical.
• Fix small typo in readme.
• Minor updates to .gitignore to make using CLion as an IDE easier

This PR should be squashed and merged. I had to remove libcec in one commit and add the submodule in a second commit.

Purpose
It will be easier to know the source of the third party code. Updates could also get easier if dependabot or renovate bot is added.

Additional Info
I was also going to add https://github.com/aizvorski/h264bitstream as a submodule, but I noticed you have a custom version of https://github.com/moonlight-stream/moonlight-embedded/blob/master/third_party/h264bitstream/h264_stream.c

For ilclient I didn't find an official git location for the library.

[ReenigneArcher]

I don't believe this failure is caused by my changes... or maybe I missed something?

23a6e5471cec: Pulling fs layer 
4a26899c0f92: Pulling fs layer 
393d2addb2a5: Pulling fs layer 
Status: Downloaded newer image for cgutman/moonlight-embedded-packaging:rpi-buster_4deffd7c9f2e190e
docker.io/cgutman/moonlight-embedded-packaging:rpi-buster_4deffd7c9f2e190e
Using pre-built Docker image - cgutman/moonlight-embedded-packaging:rpi-buster_4deffd7c9f2e190e
WARNING: The requested image's platform (linux/arm/v7) does not match the detected host platform (linux/amd64/v3) and no specific platform was requested
Checking out e50a722f69d58f28d3d950a017ba1ffa4b9e6af7
Cloning into 'moonlight-embedded'...
fatal: reference is not a tree: e50a722f69d58f28d3d950a017ba1ffa4b9e6af7
Command exited with code 128
Build failed