20210624 dev build

*** TIMING BROKEN, DO NOT USE ***

Changes since the last release:

• improved frequency of generated sound output (#36)
• improved bus values from unhandled ports (fixes KEDisk)
• improved aspect ratio (59:54) to match signal details
• changed visible areas to better reflect what can be seen on a TV
• renamed borders option to visiblearea
• added alternate debugger display mode to give full refresh (#46)
• restored -autoboot command-line option (#49)
• removed prompt to save changes to disks
• fixed border artefact colour if using upper 8 colours
• fixed incorrect zoom handling when fullscreen/minimised/maximised
• fixed missing allophone data file in non-Windows installations
• fixed %s shown in disk insert message [SDL]
• fixed current debugger trace entry colour
• changed to more accurate Z80 CPU core
• changed to pull SAASound from official project at GitHub