Custom Overlays don't honor stacking of widgets

[TimPurdum]

CustomOverlays use the same corner positioning system as Widgets, but they don't currently honor the stacking rules, so if you put a CustomOverlay and a widget in the same corner, the overlay will be hidden behind the widget. This is caused by us rendering the overlays in a separate absolute UI div. This is also the case for the GeoBlazor-only GraphicsLegendWidget.

1. Change CustomOverlay to inherit from WIdget.
2. Update the TypeScript code to move Overlays and custom GeoBlazor Widgets into the same UI divs as the Esri widgets.