Updates the used actions to their latest versions, and update the images to the -latest tags.
This currently means Ubuntu will use 20.04 LTS (and soon 22.04 LTS) and macOS will use macOS 12.
Also adds a workflow_dispatch trigger to the main workflow (the others already have it).
Publish AboutThisBuild.txt for AppImage builds.
Make AppImage name consistent with Windows builds.
Port Windows build enhancements to AppImage build.
Cache AppImage tools in build workflow.
Fix AppImage publishing script.
Remove "WinVista" from build artifact name.
