Ensure that widget IDs can be reused.

[freakboy3742]

Reworks the widget registry so that widget IDs can be re-used.

Instead of maintaining both an app-level and window-level registry, a single registry is maintained on the app, with widgets being added or removed as a result of assigning a the widget to the window. The window-level registry is then a filtered proxy over the main app registry. This avoids the problem reported as #2190 where the WeakRefDict at the App level wouldn't be cleared until a widget was garbage collected. Since there is no step where a widget is explicitly removed from the app, there is essentially no trigger (other than garbage collection) that could be used to purge from the app registry.

This means that a widget is not available for ID lookup until it has been assigned to a window as content. This is a backwards incompatible change, but one that aligns ID-based lookup with the DOM getElementByID() method.

The implementation involves some minor changes to the order in which window and app are modified:

• A widget's app must be assigned before window. This ensures the window assignment can find the app registry to register the widget.
• A widget's window must be removed before app. This ensures the window can find the app registry to deregister the widget.
• A widget's app and window must be assigned before any DOM-related operations (e.g., setting parents) are performed. This ensures that in the case of a duplicate ID, the widget isn't left in a partially valid state (e.g., where a parent has been assigned, but it's not part of a layout or part of the registry)

These shouldn't affect anything in normal operation, as assigning window and app shouldn't generally be something the end-user is doing.

Fixes #2190.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct