feat: support FreeBSD Operating System (gtk only)

[jdhg-orbiware]

Disclaimer: I don't use rust to code but saw that this change can be pretty straight forward. I do use FreeBSD. This PR works for me to compile muda on FreeBSD.

README file has been updated documenting the support and dependencies needed.

I do appreciate any feedback if there is something wrong with the PR.

Thanks in advance.

[delleceste]

muda-freebsd.patch

This is the patch I used to build muda on FreeBSD in the context of the qbz application

(https://github.com/vicrodh/qbz)

[amrbashir]

Thank you, can you extend this PR to also support other variants of unix systems

#[cfg(any(
  target_os = "linux",
  target_os = "dragonfly",
  target_os = "freebsd",
  target_os = "netbsd",
  target_os = "openbsd"
))]

[jdhg-orbiware]

@amrbashir sure, but I was able to test only in FreeBSD. Maybe later?

[amrbashir]

no need to test, we can fix them later if anyone has issues but I doubt it, thanks

[jdhg-orbiware]

@amrbashir PR updated, thanks for the feedback.