fix typo: filesSelected has been renamed file_selected

tomwer/gui/stacks.py

@@ -111,7 +111,7 @@ class _ImageStack(qt.QMainWindow):
             dialog.close()
             return
 
-        for folder in dialog.filesSelected():
+        for folder in dialog.files_selected():
             assert os.path.isdir(folder)
             self.addLeafScan(folder)