Implement Widget customization

[krschau]

Summary of the pull request

Instead of faking widget customization by creating a new widget in the old one's place, do it for real with the new IWidgetProvider2 interface.

Note: this will remove the Customize menu option for all widgets that have not yet implemented the new interface.

• Update Dev Home to use WinAppSDK 1.4, so we have access to the new API
• Move customization experience to be in-place, rather than in a pop-up

References and relevant issues

Detailed description of the pull request / Additional comments

• When customization is selected, Dev Home uses the new Widget.NotifyCustomizationRequestedAsync() API to notify the widget provider. The Widget provider will receive a OnCustomizationRequested event.
• Add "Save" and "Cancel" buttons during editing experience.

Not yet implemented:

• Localization for new buttons

Here's the SSH keychain:

Edit: this will now show the previously selected file:

Validation steps performed

PR checklist

• Closes Dev Home does not support Windows Widget IsCustomizable package manifest value #1713
• Tests added/passed
• Documentation updated