Fixed some UI issues that entered from macOS mojave / dark mode

Can now upscale @1x images (have to have @1x in the filename) to @2x and @3x and fixes #8

Uses the nearest neighbour algorithm.

Improved the UpdateModel to now check for updates via the GitHub Api, uses the GitHub release assets for downloading the new version.

Refactored the code, much better now. Also fixed an issue where if the JPEG image quality was not 10/10 it would be really bad.

Fixed the issue where a 3x file was not correctly resized to 1x on Retina Macs.