Model Extension Definitions (MEDs) Need To Be Marked As Either Built-in Or User-Defined

MEDs contributed via an Eclipse extension are built-in and cannot be removed/unregistered from the Model Extension Registry. However, user-defined MEDs are contributed by users and can be unregistered. In the current MED implementation there is no way to tell if it is built-in or user-defined. The Model Extension Registry View will have the ability to unregister MEDs so it needs to be able to find out which type of MED it is.