Allow skins to register sprites through addIconSprite
Currently, extensions can register sprites using t3lib_spriteManager::addIconSprite(). This requires an array of icons (which will be added to $TBE_STYLES['spritemanager']['spriteIconsAvailable']) and the CSS filename which contains the generated sprite.css.
A skin might also want to provide a different sprite, but the skin's CSS files are already loaded automatically from the registered directory. See #23266 where t3skin wants to add a sprite for flags. After this fix, we will use the API to do so in t3skin, simply not passing the CSS filename, but just the array of images.
(issue imported from #M16007)