Motivation for this change

I recently bought a Canon 7D DSLR and with the help of https://www.magiclantern.fm/ it can shoot RAW video in the (magic lantern) MLV format. To actually use the MLV files requires post processing. MLV App is quite popular for this. We didn't have it so I packaged it.

What is MLV App? Lightroom, but for Magic Lantern MLV Video (and open source and cross platform)

Things done

Added mlv-app. I haven't done too much testing yet since I just got the camera and haven't done a lot with it yet but so far so good. If I get an installation of macOS working again I'll try and add the macOS version... since I need to do that for a few other packages I maintain anyway.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.