AppImageShareDaemon: Advertise in LAN with Zeroconf

appimaged (*or a new AppImageShareDaemon* -- insertion by -kp-) could advertise itself in LAN with Zeroconf so that other appimaged instances could show applications residing on other machines in the same LAN, too.

One could even have a dedicated appimaged instance on an "AppImage server" (e.g., a Raspberry Pi). This would be a nice showcase for AppImages running from network shares.