Widgets and WidgetContainers vs frontend-plugin-framework PluginSlots

[MaxFrank13]

Part of this app has support for the idea of "widgets" and "widget containers" that operate in a similar way to DirectPlugins in the frontend-plugin-framework (FPF). The widget implementation pre-dates FPF so perhaps it's worth revisiting whether or not we should keep supporting this pattern.

The README details instructions on how to use the widgets pattern but also indicates support for FPF. It would probably make sense to just pick one implementation.