fix: Use self-defined logging level instead of org.slf4j.event.Level

[slarse]

#3775

This PR introduces a self-defined LogLevel enum that can be used to get or set the logging level. The reason we need this is that org.slf4j.event.Level makes Spoon backwards-incompatible with libraries that uses slf4j-api < 1.7.15. Which is not good. I've tested this locally and it fixes the test errors in Casper, as mentioned in #3775.

This is a breaking change, as the return type of Environment.getLevel() is changed. #I purposefully changed the name of the type from Level to LogLevel to make this more evident. #3755 was in fact also breaking, as the org.slf4j.event.Level is not the same as the previous Level type from log4j. With this change, however, we won't have breakage again, as we now define the type ourselves.

I have two more thoughts on this.

1. Do we want to implement this breaking change now, or should we wait for Spoon 9? If we don't want to do it now, we must also revert refactor: use slf4j API instead of log4j-api #3755.
2. As we're breaking the API anyway, we may also want to consider renaming Environment.get/setLevel() to Environment.get/setLogLevel(). These names are more descriptive ("level" is obvious in a logging framework, but not in a metaprogramming library), and also make the breaking change more obvious.

[slarse]

@monperrus per our discussion, changed LogLevel to Level.

[monperrus]

Perfect, LGTM, will merge.

[monperrus]

Very important follow-up of #3775, thanks very much.