itemBuilder property
Each existing DashboardItem in the dashboardItemController must added to widget tree.
itemBuilder callback takes a argument of DashboardItem.
DashboardItem.itemLayout carries the latest ItemLayout.
itemBuilder must return non-null Widget instance. The callback will be
called when widget created or item editing started (for only editing item)
or change item layout (for only changed items).
cacheExtend determines when the widget will creating / removing in the widget tree.
Implementation
final DashboardItemBuilder<T> itemBuilder;