[Bit-dev] Document weak dependencies when spawning sub processes?

I have just managed to identify a bug preventing the sys tray icon to be shown (together with the issue reporter) https://github.com/bit-team/backintime/issues/1236#issuecomment-1324079326 and it turned out that a required CLI tool (`xdpyinfo`) was not installed which is used to check if X11 is running https://github.com/bit-team/backintime/blob/713590f45cb83d1192a104ec77b73562... https://github.com/bit-team/backintime/blob/713590f45cb83d1192a104ec77b73562... I think we should work through all source files and document required CLI tools. This is not only required to prevent bugs but also for package maintainers.