Change Widget Pin flow to show preview images

[krschau]

Summary of the pull request

Change Dev Home's flow for pinning widgets to align with the Windows Widget Board. In the Add Widget dialog (aka widget picker), the user is shown widget preview images, and any configuration is done inline on the Dashboard.

References and relevant issues

Detailed description of the pull request / Additional comments

• Instead of widget creation happening in the "Add Widget" dialog, we only create the widget after the widget has been pinned. This lets us simplify the Add Widget dialog, since we're only selecting a WidgetDefinition there. We no longer have to specially handle the popup or window closing, because there is no "partially created" widget to delete.
• Since widget customization is done in-line, Dev Home as a widget host no longer needs to know if the widget is in a "configuration state" or not, so that can be removed from both host and provider.
• Since there is no longer a different between "add" configuration state and "edit" configuration state, we can always show the Save and Cancel buttons in the Configuration template, although we only enable Cancel if there was once a valid state we can return to.
• Added AddWidgetViewModel to show WidgetDefinition information in the Add Widget dialog.
• Added the WidgetScreenshotService to the Dashboard, since we need to show the widget preview images ("screenshots") in the Add Widget dialog. We cache the screenshots, but only when they are going to be shown.

PR in Draft while we wait for screenshot assets.

Validation steps performed

PR checklist

• Closes Change Pin Widget flow #2109
• Tests added/passed
• Documentation updated