Modernize widget router [5.3.0-B1]

The WidgetRouter router example class included with Custom module is a bit outdated. We should:

• use "?page=N" format to specify list page instead of "/N.html" format
• create AutomaticFilename and Filename fields in "widget" unit (+ show on editing page) to allow manual filename specification
• update "widget:ItemLink" tag to pass filename in "widget_filename" parameter to avoid extra filename query

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