Port to MacOs / Add drop support in the main menu

[MarkusJx]

• Bump the version of 'electron-builder' to 21.2.0, since this is the minimum required version for MacOs (Catalina)
• Instead of command-line arguments, use app.on('open-file', (event, path) => {...} to get files to be opened at program start. General idea described here.
• Set titleBarStyle: 'hidden', to hide the original title bar, but to keep the 'close', 'minimize' and 'maximize' options in the custom title bar
• Use ipc to tell the main process, when the web page has finished loading to keep the the 'Open With' option working when the program is already running
• Add 'mac' build option