v0.23.0

Full Changelog: v0.22.0...v0.23.0

Save states have associated screenshots that are shown in ingame menu.
Small bug fixes.

feat: add memory usage to about page
fix: remove disk usage from about page
lang: fix British colour spelling
feat: use lanczos3 for screenshot scaling
feat: show save state screenshot in ingame-menu
feat: save screenshot when saving state
fix: going back from subfolders goes to top (#25)
docs: update README.md