Build dedicated ecdar.app for macOS

Currently, the readme writes

> First, download the Zip archive and extract it to where you want the project to reside. Then execute the binary (ecdar on Linux or Mac and ecdar.bat on Windows). You are now running ECDAR!

While technically one could execute the binary on macOS, one runs very quickly in 'security issues', as the binary is not signed by an Apple trusted developer. Since the ecdar binary calls other binaries, it's very difficult to get all the security permissions right (I haven't been able to run the ecdar binary successfully). For this it might be better to build a dedicated ecdar.app, which a user can then give once the right security permission.