Generalize the widget for more frameworks

The idea is to widen the spectrum of the project to include more logging services, not only `spdlog`. This may require some fundamental changes in terms of splitting the project apart to have some core, shared functionality and specific front ends (in this case, the logging frameworks).

First to be done is to explore the available frameworks, gather information which works how, and decide on the common parts.

No files, tests, or entry points are named. Start by exploring available logging frameworks and how they differ from spdlog, then identify functionality that could be shared across framework-specific front ends. Done means the supported frameworks and common project structure have been investigated and a direction has been decided.

Written by the indexing model from the issue text.