fix(gioui): disable file load / save dialogs in browser

This at least gets the program to compile into js as sqweek does not work in browser, but we really need to look for a long term solution. How to load & save songs in browser?

tracker/gioui/files.go

@@ -1,3 +1,5 @@
+// +build !js
+
 package gioui
 
 import (

tracker/gioui/files_js.go

@@ -0,0 +1,9 @@
+package gioui
+
+func (t *Tracker) LoadSongFile() {
+	// TODO: how to load songs in browser, just disabling it for now
+}
+
+func (t *Tracker) SaveSongFile() {
+	// TODO: how to save songs in browser, just disabling it for now
+}