Early support for macOS

[fathonix]

Arqiver are already buildable on macOS without any modification, but lacks an icon and a list of associated files to make it show up on the "Open With" menu. This PR addresses those issues.

TODO

• [ ] Add more supported files in Info.plist
• [ ] Change qmake and CMake files to set the right version (and maybe bundle identifier) in Info.plist (currently only qmake)
• [ ] Add support for opening files (Info.plist only registers supported files but the file isn't actually opened)
• [ ] Append the executable directory to the process' PATH env on macOS to detect 7z executable if bundled
• [x] Use Qt native method or the open command instead of gio on macOS