Make the default suggested action behavior to not show a preview.  Only override that for fixes/refactoring.

Fix for XAML Dev15 RC issue #14565.

This change makes 'Workspace.DocumentActiveContextChanged' public and
introduces DocumentActiveContextChangedEventArgs to hold the relevant
set of arguments. All of its usages have also been updated.

This also introduces a breaking change to
RaiseDocumentActiveContextChangedEventAsync (a protected method of
Workspace).