Add usage statistic for unsupported mandatory field types in created content

Add a usage statistic event when the user follows the alternative step 2 of Create Content (i.e. clicks the 'Create' button in step 1 of Create Content and gets the old Wicket-based content editor). This happens when the document type contains mandatory fields that are not supported yet in Visual Editing.

Name of the usage statistics event: "CreateContentUnsupportedFields"

The event has a single parameter called "UnsupportedFieldTypes". Its value is an alphabetically sorted, comma-separated string of field type names of mandatory fields in the document type that is being created that are not supported yet by Visual Editing. Use the JCR type name when the field is a known Hippo type (i.e. part of the core CMS or of a known/supported plugin). Use the label "Custom" for all other, unknown field types.

For example, the parameter could be

UnsupportedFieldTypes="Boolean,Custom,hippogallerypicker:imagelink,hippostd:date,selection:BooleanRadioGroup

to indicate that the created document's type contains several mandatory fields that are not supported yet by Visual Editing: a boolean, some custom field, a gallery picker, a date, and a boolean radio group from the Selections plugin.