main/postmarketos-ui-plasma-bigscreen: install and run NymphCast by default

NymphCast is a FOSS Chromecast replacement (not a drop-in one though) and allows you to "cast" audio and video to your device running the nymphcast server. Plasma Bigscreen seems like an ideal use-case for this so let's install and launch it by default. People can always uninstall it anyways