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/abstractfile.js

@@ -4,6 +4,21 @@ function readFile(file, arrayBuffer, encoding){
 	reader[arrayBuffer ? "readAsArrayBuffer" : "readAsText"](file, encoding)
 	return promise
 }
+function filePermission(file){
+	return file.queryPermission().then(response => {
+		if(response === "granted"){
+			return file
+		}else{
+			return file.requestPermission().then(response => {
+				if(response === "granted"){
+					return file
+				}else{
+					return Promise.reject(file)
+				}
+			})
+		}
+	})
+}
 class RemoteFile{
 	constructor(url){
 		this.url = url
@@ -54,6 +69,23 @@ class LocalFile{
 		return Promise.resolve(this.file)
 	}
 }
+class FilesystemFile{
+	constructor(file, path){
+		this.file = file
+		this.path = path
+		this.url = this.path
+		this.name = file.name
+	}
+	arrayBuffer(){
+		return this.blob().then(blob => blob.arrayBuffer())
+	}
+	read(encoding){
+		return this.blob().then(blob => readFile(blob, false, encoding))
+	}
+	blob(){
+		return filePermission(this.file).then(file => file.getFile())
+	}
+}
 class GdriveFile{
 	constructor(fileObj){
 		this.path = fileObj.path