Managed FTFGenerator and generated bitmap font

Migrate FreeTypeFontGenerator to an asynch Loader.
-  Allow async loading of BitmapFont from TTFGenerator.
-  Configure required size with precision

Use like this :

```java
manager.setLoader(FreeTypeFontGenerator.class, new FreeTypeFontGeneratorLoader(resolver));
manager.setLoader(BitmapFont.class, new FreeTypeFontGeneratorBitmapFontLoader(resolver));

FreeTypeFontParameter parameters = new FreeTypeFontParameter();
parameters.characters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 -+";
manager.load("btnFont", BitmapFont.class, 
        new FreeTypeFontGeneratorBitmapFontParameters(
                FONT_NAME, parameters, 
                new FreeTypeFontParameterConfigurator() {
                    @Override
                    public void configure(FreeTypeFontParameter parameters, FreeTypeFontGenerator generator) {
                        parameters.size = generator.scaleToFitSquare(Gdx.graphics.getWidth() / 2 - 3 * headerPadding, labelSize, 8);
                    }
                })
);

...

buttonStyle.font = manager.get("btnFont", BitmapFont.class);
```