Fix various UX and emulation accuracy isssues.
With particular thanks to Chris Evans and Rich Talbot-Watkins.
- fix various emulation issues: teletext, CRTC, VIA, Master RTC, Master CPU
- fix problems with hi-DPI screens on macOS Catalina
- improve debugger hex editor
- improve keyboard-related UX: better default layout, improved editing UI
- improve automated tests (mostly not interesting, but should help head off regressions in future...)