Add instance capability for dataformview and dataformfield plugins.

As of recent enhancement dataformview and dataformfield plugins can be enable/disabled and uninstalled. But where a plugin is enabled it is always instantiable. There may be cases where we would want to prevent certain individuals or roles from adding an instance of a field or a view type in a specific dataform activity, a course or throughout the site. For instance, if we have a field plugin that allows the creator to specify an sql query, we may want to prevent anyone but admins from instantiating it so as to protect the server from bad queries.

This enhancement will add the addinstance capability to all dataformview and dataformfield plugins that can be instantiated. The capability is required for making the plugin instantiable. Internal field types such as the entryactions, entrygroup, entryauthor and entrytime are not instantiable and do not need this capability.

The capability will be checked by when rendering the plugin selector in the view and field index pages and will be required in the view and field edit pages. This should ensure that plugins that cannot be instantiated not only omitted from the selector but also cannot be added via direct url to the edit page.