Add MDNS discovery

Equivalent to the Android MR here: kdeconnect-android!375 (merged)

The new behavior is behind the CMake flag MDNS_ENABLED (off by default).

The env var KDECONNECT_DISABLE_UDP_BROADCAST can be used to disable the regular UDP broadcasts, for manual testing.

This implementation uses KDNSSD, which only supports Avahi so it's Linux-only. I'll add support for Windows and MacOS in a separate MR.