Added a FreeTypeFontGeneratorLoader

A loader for the `AssetManager` so the generators can be managed just like other assets.

If this gets merged I'll add a little note to https://github.com/libgdx/libgdx/wiki/Gdx-freetype about how to register this loader if the gdx-freetype extension is used.

I created this via the github website. I hope there's no typo anywhere...