SystemFontSelector is a DropdownMenu widget that displays a list of all the available fonts in the system.
The user can select a font from the list and the font will be loaded and font name will be passed to the onFontSelected callback.
All the font names can be showed in their fontfamily if isFontPreviewEnabled is set to true but this will cause every font to be loaded.
Remember every font can be loaded once and if loaded, font will be available for app lifetime,
so if you somehow loaded every font once you can use font preview without any additional cost in every SystemFontSelector instance.
