Trim extensions when sorting filenames

[sarayourfriend]

Fixes #518

I've described the root cause in this comment on the issue: #518 (comment)

To test:

• Checkout the branch and rename some of the demo images to 10, 10.1, 10.2, etc.
• Run the app and confirm that they now sort as expected according to the issue (10, 10.1, 10.2 rather than 10.1, 10.2, 10).
• Download a PNG (here's a small filesized CC0 one from Wikimedia Commons you can use and name it 10.png in the demo images folder. Reverse the name sorting a couple times and confirm that 10.jpg and 10.png correctly trade places and sort alphabetically by their full filename. This test is crucial to confirm consistency in the name output, otherwise the identical extensionless names would make sorting them non-deterministic.

[bpatrik]

Thank you for the contribution!