Filter shown widgets to those from packages that also implement the Dev Home extension

[krschau]

Summary of the pull request

We want to open up the widgets we show as available to the user, but we still want to limit the widgets to ones specifically meant to be shown in Dev Home. So instead of filtering on widget provider name, instead we will show widgets that also implement the Dev Home extension.

We do this by finding all installed packages that implement com.microsoft.windows.widgets' in the same way we find those that implement com.microsoft.devhome`. In order to do this, we need to add com.microsoft.windows.widgets as a AppExtensionHost in the appxmanifest. Then we can take the intersection of the two lists, and a list of the PackageFamilyNames of that intersection.

We use the PackageFamilyName since that will match the first part of the Widget Provider's Id, so we can compare that Id to the list and know that the widget came from a package we want to include.

References and relevant issues

Detailed description of the pull request / Additional comments

Validation steps performed

PR checklist

• Closes Custom widgets require a Microsoft package ID #908
• Tests added/passed
• Documentation updated