MenuCssClass is used to add css to the Menu node. The css allows for
replacing menu with an icon and other super-fun and helpful things.
cssClass is a StringFunc which can either be a String constant or
a Function that returns a String. Thus, you can compute the
css based on the current state or you can have a constant. Syntactically
you can use either:
MenuCssClass(() => calculateCssForMyMenuItem())