Use native file dialog save on Desktop

We now instantiate a file save dialog to allow saving wav files on
Desktop.

scenes/main.gd

@@ -107,8 +107,23 @@ func _on_wav_button_pressed() -> void:
 	var proposed_fname := "morse-" + Time.get_datetime_string_from_system(true) +  ".wav"
 	match OS.get_name():
 		"Linux", "macOS", "Windows":
-			wav.save_to_wav("/tmp/foo.wav")
-			print("GLobalized path: " + ProjectSettings.globalize_path("user://morse.wav"))
+			print("Save on ", OS.get_name())
+			
+			# create file dialog
+			var fd := FileDialog.new()
+			fd.title = "Save Wav file"
+			fd.use_native_dialog = true
+			fd.add_filter("*.wav", "WAV files")
+			fd.current_file = proposed_fname
+			
+			var save_method := func _save(path: String):
+				print("Save as ", path)
+				wav.save_to_wav(path)
+				
+			fd.file_selected.connect(save_method)
+			
+			add_child(fd)
+			fd.popup_centered()
 		"Android":
 			print("Sharing file on android")
 			var tmp_file_path := OS.get_user_data_dir().path_join(proposed_fname)