Logging library

[rmu75]

In #4627 it came up that we do want a more modern logging facilty. This discussion should be the place to gather requirements and possible candidate libraries. This concerns non-realtime user-space stuff like setup/tear down code, GUI and language interfaces, message pumping/processing, task and similar stuff.

I (@rmu75) would want interface with fmtlib style type safe formatting, some shortcuts to specify severity level and possibility of central configuration of log targets (like file, stderr, GUI, whatever).

No-effort googling yields spdlog https://github.com/gabime/spdlog as candidate (also packaged in debian).