Old behavior

Every plugin had to setup SongInfo separately

Which causes duplication of listener registration win.on("page-title-updated"... and ipcMain.on("song-info-request"...

Which causes song-info callbacks to be called multiple times
(because ipcMain callbacks were registered X times and sent X events for each "song-info-request")

New behavior

songInfo is setup once from index.js before plugin load.
Each plugin can register a song-info callback by just requiring it. (without the need to set it up)

achieved by separating registerProvider() and registerCallback() in song-info.js

registerCallback example:

const registerCallback = require("../../providers/song-info");
registerCallback(songInfo => {
	console.log(songInfo.title);
})

This fix also warranted a small refactor of notifications plugin (which was the most heavily affected by this bug)