Add usage statistic for unsupported field types in Visual Editing

Add a usage statistic event when the user edits a document in the Visual Editor and the document contains fields that are not yet supported (indicated by the grey message on top "Not all fields of this document are shown").

Name of the usage statistics event: "VisualEditingUnsupportedFields"

The event has a single parameter called "UnsupportedFieldTypes". Its value is an alphabetically sorted, comma-separated string of field type names in the type of the edited document 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 edited document's type contains several 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.