Add support for Cadence SSE

[za419]

This is scope taken out of #104 because I decided it's not needed in version 1.6.0. I would still like to support it eventually, especially because it will tighten up our resource use and reduce any intervals where fetching nowplaying information shows outdated album art.

Consider allowing support to switch between SSE and polling in config, as well - In case of misbehavior or debugging needs.