Use SLF4J for logging

[koppor]

Currently System.out.println is used. Example: https://github.com/dukke/FXThemes/blob/33590f7524e607bba1faaf9cd8b3b1c5d29a9462/FXThemes/src/main/java/com/pixelduke/window/ThemeWindowManagerFactory.java#L34

Please use SLF4J - Manual at https://www.slf4j.org/manual.html

The FXThemes will use the API only - and the consumers will route to their favourite logging framework. See https://www.slf4j.org/manual.html#swapping. This also applies for testing; where a concrete logger should be weaved in.

If that feels to be too much, tinylog could be used. Then, users can still re-route to their favorite logging framework: https://tinylog.org/download/