Skin supports BitmapFonts with multiple index.

Use case:

Create a BitmapFont with various pages and pack it with TexturePacker (inside the TextureAtlas we would have index 0, index 1, ...).
When adding them to the Skin via Skin#addRegions(TextureAtlas) the index is not used so they are overriden in the map under the same key.

Solution:

Use the index to store the regions under different keys in the map.
This pr allows the Skin to be able to load BitmapFonts with multiple .pngs.

Don't know if this is the best solution for this problem.