Allow loading SpritefontsX.xnb from theme folders

[FS-21]

SpritefontsX.xnb files are placed in the Resources folder and contains the fonts the launcher will use always.

What I suggest is to support the load of these Spritefonts files inside each THEME folder if they are available like happens with image files, texts, etc.

one user-case is that Asian players can have their own fonts files and non-asian can use different. Another different use-case is to increase the theme customization because all themes are tied to the same fonts styles.