display number of files

[memeramita]

Context

34

Changes

1. I created a label under the file widget box to show the current number of files being open and a function update_num_files_label in images_view.py for updating the label.
2. The function update_num_files_label is called from images_view.py, images_controller.py, and main_controller.py when image files are added, deleted, reset, and loaded from a csv file.

[yrkim98]

• model holds file dict
• model has get_num_files function as well as getter and setter for file dict
• any references to file_dict now go through model getters and setters
• update image count each time add_image is called
• jump on a call to see how we can do events