suggestion: sort the image_list alphabetically for easier navigation

[aubreymoore]

I love digitalsreeni-image-annotator. Thanks for sharing as open source.
For my current project I need to be able to locate specific images quickly for re-annotation.
I suggest that the image_list should be sorted alphabetically by default to facilitate navigation.
I was able to do this easily on my local copy by adding a single line:

annotator_window.py
...
def setup_image_list(self):
    ...
    self.image_list = QListWidget(QMainWindow)
    self.image_list.setSortingEnabled(True)   # this is the line I added