Search for a solution to sign the installers

[McPringle]

From the website:

Important information for Windows and macOS users: The installer is not signed. If you start the installation, you'll get a warning, but you can go over it and continue the installation. GPX Animator is open source and free to use, we don't earn money with it and thus can't afford to pay the yearly fee of roundabout € 270 (for Windows) and € 100 (for macOS) for the code signing certificate! If you know a better solution, please let us know. Thank you very much for your understanding.

If we are able to find some sponsors (see #211), we can use some money to buy a certificate. But maybe there are other solutions we can use, e.g. cheaper or maybe free certificates for open source projects?