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/main.js

@@ -90,6 +90,7 @@ var settings
 var scoreStorage
 var account = {}
 var gpicker
+var db
 
 pageEvents.add(root, ["touchstart", "touchmove", "touchend"], event => {
 	if(event.cancelable && cancelTouch && event.target.tagName !== "SELECT"){