Improved variable names/Less code duplication

See the description of my commits. I am curious if this branch can be merged with the master branch, because I am not sure if the branching was executed correctly.

mvc/controller.py

@@ -41,6 +41,10 @@ class ControllerClass:
                 self.view.tf2_label["text"] = label_text
 
     def check_for_files(self):
+        """Checks whether the two files for analysis have already been
+        selected. Then it either starts the analysis or tells the user to
+        choose files.
+        """
         if self.model.file_1 is None:
             self.view.message_label["text"] = "Cannot compare: " \
                                               "No source file selected!"