Create BitmapFontGroup

This will be the new class that the assetloader will be set to load when
using freetypefontloader.

It's not possible to do Map<String, HashMap>.class, so the workaround is
to just make a specialized type that extends Map<String, HashMap>, and
use .class on that.
Update FreeTypeFontParameters so that it works with BitmapFontGroup

Previously, FreeTypeFontParameters only held a single
FreeTypeFontParameter object. We are working towards having a
FreeTypeFontLoader that can load multiple BitmapFonts from a single
FreeTypeFont, so FreeTypeFontParameters needs to be able to hold
multiple sets of parameters.

The name that is associated with any FreeTypeFontParameter will be the
name that is used to get the BitmapFont that is created from that
FreeTypeFontParameter from its BitmapFontGroup.
Update FreeTypeFontLoader to use BitmapFontGroups
Update usage of the new asset loader for BitmapFontGroups

The method makeCorbelParams() should be an adequate example of how to
load fonts with the new loader, and an example of how to get the font
that is created can be seen in either LevelScreen or LevelTitleScreen,
in setupFonts() or generateFonts() respectively.
Rename class to better reflect its purpose