Simplify meson build

[NathanBnm]

I added some documentation to meson files and removed unused constants.

I moved the icon folder into the data folder because it's more consistent.

I also simplified the post_install.py script.

And a few minor changes. Let me know if you have any questions about this PR :smile:

[NathanBnm]

@parnold-x would you please mind to review this?

[vixalien]

@parnold-x ping!