improved indicator font customization

This allows specifying the layout indicator font using the "[FAMILY-LIST] [STYLE-OPTIONS] [SIZE]" format (for example "sans bold 9").
Previous implementation with cairo_select_font_face() didn't allow for font styling nor for fallback font families.
This should provide better support for keyboard layout indicator look & feel customization. 

p.s. also added better descriptions of expected data formats to gconf schema.