All widgets within a subcategory will be rendered in the Piwik reporting UI under the same page. By default
you do not have to specify any subcategory as they are created automatically. Only create a subcategory if you
want to change the name for a specific subcategoryId or if you want to specifiy a different order so the subcategory
appears eg at a different order in the reporting menu. It also affects the order of reports in
API.getReportMetadata and wherever we display any reports.
To define a subcategory just place a subclass within the Categories folder of your plugin.
Subcategories can also be added through the {@hook Subcategory.addSubcategories} event.