Improve meson build

[NathanBnm]

I made a few improvements to your app structure. These are not breaking changes. Let me know if you have any questions 😄

Changelog:

• Renamed COPYING to LICENSE
• Improved meson.build files
• Added some comments to meson.build files
• Improved and simplified post_install.py script

I would be happy to add French translation once this will have been reviewed

I tried to build and run the app locally using both meson and flatpak and I had no issues

[Antolius]

@NathanBnm thank you for the pull request!

I see you've made a few types of changes:

1. Rename of the file containing the full license text. I'm not an expert on software licensing, but I think the official recommendation is to name this file LICENSE. I'd like to keep that official naming, unless there's some practical reason for diverging from the norm.
2. I see you expanded names of a lot of files in the data directory to include the fully qualified app id. This seems unnecessarily verbose. If possible I'd like to keep file names concise and append the full id from meson, where meson.project_name() can be used. Ideally I'd like to have the full id repeated in as few places as possible.
3. You added calls to gtk-update-icon-cache and update-desktop-database to the post install python script. Are these steps necessary for the installation on users' systems? I haven't found any mention of these in elementary's developer docs. I'd like to skip any installation step which is not necessary for actual users, as with this app I think number of installations on users' systems will greatly outweigh number of times the app is built during development.
4. You separated the build stuff from a single meson.build file into several files interspersed between different directories. This seems nice, I like this change!
5. You added the po/extra directory and moved part of the translation strings there. I never did any translation work, so I don't know much about this process. If this separation makes it easier to localize and / or translate things, then I'm all for it!