Add icon for MacOS Tahoe

[vitvakatu]

New .icon format is used in the latest MacOS, otherwise our application icon looks bad.
Compare with other apps:

The support for .icon is not available yet in the electron-builder, but it will be there soon: electron-userland/electron-builder#9279

[hubertp]

In electron-builder since 26.2.0. Upgrading the latter is a task on its own.