Improve meson build system and integration

[NathanBnm]

This PR is the first step of some improvements to your app.

This one improves the way you use the meson build system. I'll do some other changes after merging this PR (It's better to make things step by step).

Changelog

• Add AUTHORS and LICENSE (GPL 3.0) files
• Improve meson files (separation between different folders for more clarity, for example in src and data)
• Add com.github.aprilis.messenger.appdata.xml file
• Prepare your app to be translatable (po folder and .in extension to desktop and appdata.xml files for example)
• Move and improve post_install.py script

Let me know if you have any questions :smile: I like your app and I want to contribute. Hope you enjoy it!

NB: You may have to update some details in appdata files, I'm thinking about name and email that I just found somewhere in your repo. And this is about the app description too.

This PR is ready for review

[aprilis]

Wow, I didn't expect anyone to contribute so much to my project. Thank you, I'll review your PR soon