Major refactoring of ModelTrainer

[alanakbik]

This PR is mostly based on #3084 by @plonerma and executes a major refactoring of the ModelTrainer.

Why? Since Flair 0.1 a very large amount of features were added to the ModelTrainer, causing the class to become very bloated and difficult to extend/debug.

This PR refactors the trainer in several ways:

• it adds a Plugin logic for all non-essential features. Users can write new plugins that attach to the trainer through callbacks in different parts of the training. This gives users hopefully more flexibility in adding special features to the Flair trainer (like specific logic frameworks) .
• after years of "feature bloat", some little-used features were removed. This also slims down the number of parameters the trainer can take.
• similar to before, the train and fine_tune methods are "best practice" configurations for model training. A new train_custom method is added to this that gives users maximal flexibility when doing model training.

We hope that a more structured ModelTrainer will allow us to better integrate new features in the near future, such as multi-GPU and more logging support.

[plonerma]

The introduction of train_custom in combination with the other functions which offer a set of presets makes the Trainer more flexible. Overall, the train functions are much more compact.

I am a bit unsure about the events which are not currently used in any of the plugins. I can see that one might only want to keep those which are referenced by plugins in the repository. However, I already started using some of them in my experiments and think they offer a much greater potential for extending the train function (without the need to derive a trainer class with an overwritten train function).