Change all double leading underscores with sigle leading underscores.

Also make minor refactoring and stylistic changes.

clickable_progress_bar.py

@@ -10,21 +10,21 @@ class ClickableProgressBar(QProgressBar):
 
     def __init__(self, parent=None):
         """Initialize the instance."""
-        self.__text = ''
+        self._text = ''
         super().__init__(parent)
 
     def text(self):
         """Return the text displayed on the bar."""
-        return self.__text
+        return self._text
 
     def set_idle(self):
         """Set the bar to a non-downloading status."""
-        self.__text = Constants.CLICK_TO_UPDATE_STR
+        self._text = Constants.CLICK_TO_UPDATE_STR
         self.setMaximum(self.minimum() + 1)
 
     def set_updating(self):
         """Set the bar to a downloading status."""
-        self.__text = Constants.UPDATING_STR
+        self._text = Constants.UPDATING_STR
         self.setMaximum(self.minimum())
 
     def mousePressEvent(self, event):