registerWidgetSlot function
Registers builder under slot name, replacing any existing entry.
A document's widget component names the subtree it shows by slot; the
app registers its builders at startup (registerWidgetSlot('scoreboard', () => const Scoreboard())) so documents authored against them realize. A
scene loaded with an unregistered slot skips the component.
Implementation
void registerWidgetSlot(String name, WidgetSlotBuilder builder) {
_widgetSlots[name] = builder;
}