CustomSongs: Restore custom song list after reload

Uses the File System Access API supported in some browsers to keep the custom song list between sessions, restoring it back even when the page was closed.

public/src/js/idb.js

@@ -0,0 +1,51 @@
+class IDB{
+	constructor(name, store){
+		this.name = name
+		this.store = store
+	}
+	init(){
+		if(this.db){
+			return Promise.resolve(this.db)
+		}
+		var request = indexedDB.open(this.name)
+		request.onupgradeneeded = event => {
+			var db = event.target.result
+			db.createObjectStore(this.store)
+		}
+		return this.promise(request).then(result => {
+			this.db = result
+			return this.db
+		}, target =>
+			console.warn("DB error", target)
+		)
+	}
+	promise(request){
+		return new Promise((resolve, reject) => {
+			return pageEvents.race(request, "success", "error").then(response => {
+				if(response.type === "success"){
+					return resolve(event.target.result)
+				}else{
+					return reject(event.target)
+				}
+			})
+		})
+	}
+	transaction(method, ...args){
+		return this.init().then(db =>
+			db.transaction(this.store, "readwrite").objectStore(this.store)[method](...args)
+		).then(this.promise.bind(this))
+	}
+	getItem(name){
+		return this.transaction("get", name)
+	}
+	setItem(name, value){
+		return this.transaction("put", value, name)
+	}
+	removeItem(name){
+		return this.transaction("delete", name)
+	}
+	removeDB(){
+		delete this.db
+		return indexedDB.deleteDatabase(this.name)
+	}
+}