Add automatic update checker

[randshell]

This change automatizes syncing new Deezer releases, keeping the Linux version closer to the latest official release. I've also updated the current version to the latest one as of now, which is 6.0.150.

A GitHub Secret is needed for the FlatHub Bot so that it can automatically commit the new update and open a PR.

[aunetx]

Hello, thank you very much for your work, this is incredible! I wanted to do something similar for quite some time, but never knew how to...

I've got a little question (I don't know that much about github actions, sorry if that's dumb): about the github token, I created a personal access token with the name GITHUB_TOKEN with the required permissions, but I'm not sure that's enough to make this work... So I'm merging this, but if that somehow fails then we will know why!

[randshell]

Hi @aunetx! You're welcome. I was actually pretty excited about contributing here. Thank you for maintaining Deezer for Linux. 😄

Regarding the GITHUB_TOKEN, I read more about it and it turns out that it is automatically created by Github as a GitHub App on a per repo bias, once it is specified inside a workflow file. I think at this point it should be safe to delete the personal access token you created and still see if there is any fail. xD

[randshell]

Oh, I've also just found out that scheduled workflows get automatically disabled if there is no repo activity in 60 days.

If I understand it right, the bot creates a new branch and commits the new Deezer version there, which technically should be repo activity. In other words, as long as there is a new Deezer update every 60 days, this repo should require no manual intervention haha.