This commit adds mDNS [https://tools.ietf.org/html/rfc6762] advertising for the mobile application,
allowing other applications to discover kdeconnect-supporting devices in the local network without resorting to implement the proprietary discovery protocol.
Note that this is a WIP, just to get some feedback if such would be accepted. The code requires clean-up before this can be merged.
Example output from avahi-browse -ar:
= enp0s31f6 IPv6 KDE Connect on google _kdeconnect._tcp local hostname = [Android-4.local] address = [192.168.200.146] port = [1716] txt = ["version=7" "type=phone" "name=google" "id=ea29c924xxxxxxx"